Boeing 707 — Warning Systems

1. General arrangement and warning philosophy

Chapter 10.20 separates the aircraft’s warning equipment into aural warnings, visual warnings, illustrations of the audible devices, and the subsequently added Ground Proximity Warning System. The distinction reflects the cockpit design of the period: an audible signal attracts immediate attention, while individual lights and instrument indications identify the particular condition. A warning can also have its own dedicated audible signal rather than being routed through a common master alarm.

The warnings described in the Lufthansa manual span fire detection, landing gear position and door locking, takeoff configuration, engine operation, hydraulic pressure, electrical generation, air conditioning, cabin pressurization, fuel systems, navigation, autopilot, trim and flight control functions, cargo and passenger-related equipment, and terrain proximity. Many warnings are installed at the flight engineer’s station because the flight engineer is responsible for monitoring and managing major aircraft systems.

Not every Boeing 707 carried precisely the same equipment. The TFV repeatedly identifies differences between the 707A, 707B, and 707C, and also makes references to particular optional systems, including the navigation warning installation and GPWS. Therefore, the following descriptions represent the equipment covered by this Lufthansa chapter, not a claim that every individual 707 ever built had each light or warning.

2. Aural warning systems

The manual identifies two bells, one warning horn, and one chime at the flight engineer’s station. It also describes a horn-cutout arrangement on the pilot’s light shield. These units have distinct purposes; the same sound should not be assumed to indicate the same condition.

2.1 Fire alarm bell

The fire alarm bell is described as a continuous alarm. Detection of a fire condition at an engine nacelle or main wheel well causes the bell to ring continuously. The related fire-warning lights illuminate, providing a visual indication of the affected location. A BELL CUTOUT pushbutton on the pilot’s light shield silences the bell without removing the underlying warning indication. The warning is therefore acknowledged acoustically; the physical fire condition is not reset merely by pressing the button.

The manual refers to the dedicated engine fire-detection and fire-extinguishing documentation for the complete operation of the fire-warning circuits. Its warning-system chapter establishes the relationship between fire detection, the bell, the cutout control and the visual annunciators, but does not reproduce every fire-detection loop or extinguishing valve connection.

2.2 Master warning bell

A separate bell is described as producing an intermittent signal for a master warning. A master-warning relay is energized when the pilot-static-pressure-operated airspeed warning assembly produces a warning condition. The manual references the airspeed and Mach limit systems for the detailed sensing arrangement. The essential distinction from the fire alarm is that the master warning bell is intermittent and is related to the master warning/airspeed warning circuit rather than the engine fire-detection loops.

2.3 Warning horn — continuous and intermittent functions

The warning horn serves both takeoff and landing-gear-related configuration warnings. When the landing gear is in an unsafe-to-land condition, the horn operates continuously and the landing-gear warning lights on the pilot’s centre panel illuminate. Depending on gear and flap configuration, a red landing-gear warning indication is presented. The manual states that when the gear is not locked down and the pilot’s control-stand flap lever is in the landing configuration, the warning horn sounds continuously. The horn can also be triggered when any throttle is moved toward idle under specified landing-gear conditions.

For unsafe takeoff, the same horn operates intermittently. On the ground, advancing the No. 3 thrust lever into the takeoff configuration initiates a warning if the configuration is unsuitable. The manual identifies three principal conditions:

• The speed-brake handle is not in its zero position.
• The flaps are outside the permitted takeoff range.
• The stabilizer trim is outside the takeoff (green-band) range.

Permitted flap limits are aircraft-variant dependent in the source:

The source specifically indicates that stabilizer-trim positioning outside the green band triggers the horn, subject to the circumstances and trim control provisions described in the stabilizer trim chapter. The warning horn is energized from the 28 V DC system. Its intermittent operation is achieved using the horn relay and associated interrupter circuitry. The later illustrations in the chapter show the thrust-lever and flap-handle relationships and the horn-cutout lever.

2.4 Cabin altitude above 10,000 feet

A separate intermittent warning occurs if the control cabin altitude rises beyond approximately 10,000 feet. The manual describes this as an airspeed/Mach warning-related alerting circuit energized by the cabin-altitude warning switch arrangement. The warning can be removed by bringing the cabin altitude below the threshold or addressing the relevant electrical condition. This is a cabin-pressure altitude warning, not an aircraft flight-level warning: the aircraft may be cruising far above 10,000 feet while the pressurized cabin remains well below that value.

2.5 Cabin call chime

The chime is associated with communications between the cockpit and cabin. A signal received on the VHF or HF system can produce a two-note signal, while a flight-attendant-to-cockpit call causes the chime to sound continuously and illuminates the appropriate call indication. The circuit is silenced by pressing the relevant call or reset control. In the Lufthansa manual this item is categorized with audible warnings although it performs a communication-alerting function rather than indicating a system malfunction.

3. Visual warnings and their cockpit locations

The TFV divides the visual warnings according to their installation location. Red warning lamps and amber caution lamps are fitted on the pilot’s light shield, centre panel, copilot’s panel, the flight engineer’s upper and lower panels, the auxiliary and overhead panels, the forward electronic control panel, and the pilot’s control stand. Blue, green and other status indications appear at various locations but are not necessarily warnings. The source explicitly states that caution/warning lamps are distinct from position and call lights even when installed alongside them.

The distinction between red and amber is operationally important. Red generally calls attention to a serious warning condition, such as fire, unlocked landing gear, or major system failure. Amber normally identifies a caution, abnormal condition or a system requiring crew action or monitoring. However, the exact meaning is determined by the circuit and placard, rather than colour alone.

4. Pilot’s light shield: fire detection and extinguishing indications

4.1 Engine fire warning lights

Engine fire indications are located in the pilot’s light shield. When an engine fire condition exists, associated red warning lights in the engine-fire control area illuminate. The warning can be accompanied by the continuous fire alarm bell. The aircraft’s engine fire detection equipment supplies the sensing signal, while the light-shield installation provides the immediately visible crew indication.

4.2 Main wheel well fire

A dedicated red warning light labelled MAIN WHEEL WELL is provided. It illuminates for a main wheel-well fire detection condition. The same condition is associated with the fire alarm bell; engine and wheel-well fires are differentiated by their illuminated annunciators.

4.3 Fire warning test arrangements

The TFV describes test controls whose behaviour varies by aircraft variant. In the 707A arrangement a fire test can exercise fire-warning indication, the fire alarm bell, and the related fire detection circuit. For 707B/707C installations, the described fire-test arrangement employs distinct switch positions for engine-fire and discriminator testing, while the fire-extinguishing test circuits have their own indication provisions. The text explicitly separates the simulation of a detector condition during testing from an actual engine fire.

4.4 Extinguishing circuit lights

The light shield also contains SHORT CIRCUIT warning indications for fire-extinguishing control circuits and BOTTLE DISCHARGE lights for the extinguishing bottle discharge circuits. The 707B/707C source describes amber short-circuit lamps that illuminate when the corresponding extinguishing fire-detection or control wiring is affected. Bottle discharge lamps are identified by bottle number and system arrangement. Their presence should not be confused with the engine-fire detection lamps themselves: they report the status or condition of the extinguishing equipment rather than detection of flames in the engine nacelle.

5. Pilot’s centre panel warnings

5.1 Landing gear unsafe

The red GEAR light is illuminated if any landing gear is not locked in the commanded position. The source describes the condition with reference to the landing gear control lever in the UP or DN detent and the gear’s locked status. A warning also exists when the gear is not down and locked but the flap handle has been moved toward a landing configuration. This indicator must be distinguished from individual landing-gear position lights, which separately indicate gear status.

5.2 Landing gear door warning

A red DOOR warning light illuminates when any landing gear door is not closed and locked. It is an independent warning of the door locking condition; a door can be improperly secured even if the main gear indication appears otherwise acceptable. The detailed sensing logic belongs to the landing gear door-warning circuit.

5.3 Master warning light

The pilot’s centre panel includes a flashing red master warning light labelled WARNING PUSH TO RESET. It lights when an electrical failure warning signal is received, including conditions annunciated from the flight engineer’s upper panel. The source describes the master light as flashing until reset by pressing the light. The individual warning lights remain illuminated as long as the underlying fault continues. Consequently the master warning is an attention-getting device, not an instruction that every affected system has returned to normal.

5.4 Engine overheat (707A)

The 707A section identifies four amber ENGINE OVERHEAT lamps. They illuminate when the temperature of the internal engine cooling air exceeds 450°C at the described detector locations, or when the relevant overheat switches operate. The manual directs the reader to the engine overheat system for the detailed implementation and should not be interpreted as saying that 450°C is an engine exhaust gas temperature limit.

5.5 Thrust reverser operating indications

Amber reverse-thrust control lights are installed on the centre panel. The labels and installation vary between the 707A and 707B/707C variants, including references to REVERSE THRUST and REVERSER status. Their purpose is to indicate reverser operation or position in accordance with the respective thrust reverser control system. They are not equivalent to a command to select reverse thrust.

5.6 Engine oil pressure warning

The OIL PRESSURE warning lamps supplement the engine oil pressure gauges. They provide a low-pressure indication when engine oil pressure falls below the specified threshold. The TFV points to the power plant system for the detailed pressure limits; this warning chapter does not establish a single universal oil-pressure set point for every engine version.

5.7 Mach trim disengaged

An amber MACH TRIM DISENGAGED annunciator is provided. The indication is associated with the Mach trim system being inoperative or disengaged. The source describes a test position and remarks on activation conditions. In a historical simulation, this light should follow actual Mach trim availability and engagement logic rather than a generic “autopilot off” condition.

5.8 Autopilot disengaged

A red AUTO-PILOT DISENGAGED lamp flashes intermittently after autopilot disengagement, including a disconnect made by the pilot or a disconnect due to a fault. The indication can be reset by the appropriate control. The TFV makes a further distinction: when the yaw damper is affected by operation of the override switch, the light may illuminate steadily rather than using the same flashing warning sequence. Autopilot and yaw damper indications should therefore not be implemented as an undifferentiated warning.

6. Copilot and navigation-related indications

Where the optional navigation warning system is fitted, a red NAV WARNING PUSH TO RESET master navigation annunciator is present. A navigation warning is triggered by a fault affecting the comparator or applicable navigation indications, with details depending on the equipment installed. A separate amber HORIZON comparator lamp is identified on 707A/707B aircraft without the later navigation warning system. The source describes it as indicating a discrepancy between attitude indications on the captain’s and first officer’s flight instruments, including indications on the flight-director and artificial horizon systems.

The copilot’s panel includes references to master warning and horizon comparator lights as applicable. It also includes a red RUDDER BOOST LOW PRESS warning lamp. That indication is associated with insufficient rudder boost hydraulic pressure and is described as being triggered when hydraulic pressure drops below the applicable operating threshold. The manual’s example refers to pressure monitoring around 1,450 psi in the rudder hydraulic system; the aircraft-specific hydraulic documentation should be used when implementing the exact warning switch operation.

Other cockpit warnings referred to in this location include low hydraulic pressure lamps and cargo, galley and entry-door warning indications that are mounted on the flight engineer’s panel rather than directly in the copilot’s primary instrument area.

7. Flight engineer’s upper panel

The flight engineer’s station provides the principal warning annunciation for the major aircraft systems. Several lamps at this location concern electrical supply integrity and can feed the cockpit master warning circuit.

7.1 Essential power failure

A red FAILURE lamp warns of an essential-power supply failure. The manual describes the warning as being driven through the essential bus power monitoring circuit and associated relays. This is significant because the 707 electrical system distinguishes generator supply, bus tie status and essential power selection; failure of an essential supply path is not necessarily identical to a failure of a particular generator.

7.2 Generator drive low oil pressure

Four amber LOW OIL PRESS lamps are associated with the constant-speed generator drives. They indicate a low oil pressure condition in the individual drive units. The source gives a threshold of approximately 30 psi for the generator drive warning in the described installation and explains that the lamps are driven by oil pressure switches. Loss of oil pressure in the generator drive is a different fault from low oil pressure in a JT3D engine.

7.3 Bus tie breaker open

Four amber CIRCUIT OPEN indications identify the corresponding bus-tie breaker or generator circuit configuration. Their function is to show an open connection in the AC distribution arrangement. In an accurate systems simulation these lamps should follow the actual bus-tie state and power configuration, including intentional opening, not simply whether the engine is running.

7.4 Generator breaker open

Four amber CIRCUIT OPEN lamps are also associated with the generator breakers. These indicate the opening of the relevant generator circuit breaker. The manual makes a distinction between generator control operation and breaker position; both must be interpreted in the context of the AC electrical system.

7.5 Generator control

The four amber OFF generator control lamps indicate the status of the generator control relays. Their use allows the flight engineer to distinguish a generator made unavailable by its controls from a generator that is operating with its breaker open or a bus tie that is disconnected.

7.6 Turbocompressor oil pressure

Three amber LOW OIL PRESS lamps are assigned to the air-conditioning turbocompressors. They operate when pressure at the associated turbocompressor falls below approximately 55 psi. Separate warning lamps are required because the turbocompressors are part of the aircraft pneumatic and air-conditioning equipment, not the engine lubrication system.

7.7 Turbocompressor overspeed

Three amber OVERSPEED TRIP lamps indicate that the relevant turbocompressor has been tripped by overspeed protection. The source associates the overspeed operation with the appropriate turbocompressor control circuitry. A trip indication is distinct from a low-oil-pressure warning and should remain tied to the actual protective shutdown logic.

7.8 Air-conditioning unit overheat

Amber OVERHEAT warnings are provided for the air-conditioning units. Their temperature sensing is associated with the air-conditioning machinery and the corresponding overheat switches. The source lists a compressor outlet temperature threshold of approximately 170°C for the relevant ACM warning circuit. The precise temperature is tied to this unit and must not be transferred indiscriminately to cabin temperature or bleed air temperature indications.

7.9 Air-conditioning unit OFF

Additional amber OFF lamps identify an air-conditioning unit that has been shut off or disabled by the pertinent control logic. This is operationally different from an overheat indication, although an overheat may cause an associated automatic shutoff.

7.10 Duct overheat and heater CLOSED status

The source describes amber DUCT OVERHEAT warnings when duct air temperature reaches the specified overheat condition, as well as amber CLOSED lights for the applicable cargo heater outflow valves. The warning chapter states an approximately 90°C duct overheat condition for the described installation. The heater CLOSED indication, however, is a valve-position status indication, not proof that the duct is overheating.

8. Flight engineer’s lower panel

8.1 Fuel de-icing indications

The TFV differentiates the fuel anti-icing/de-icing arrangement by aircraft version. For the 707A, amber ICING lights indicate that the fuel temperature has dropped below the normal controlled value, with a described nominal switching value near 4.4°C. For 707B and 707C aircraft, additional amber ICING indications are associated with the fuel filter restriction or differential-pressure arrangement. The manual states a differential pressure value of approximately 4.5 psi in the description of the filter installation. Consequently the meaning of an ICING annunciator depends on the installation and must be distinguished from a general wing or engine anti-ice warning.

8.2 Fuel heater valve open

The 707B/707C installation includes amber HEATER VALVE OPEN indications that illuminate when the corresponding fuel heater valve is open. These are system-state indications, even though listed with the caution lamps. They supplement the fuel heating and anti-icing controls.

8.3 Fuel boost pump low pressure

Fuel boost pump pressure is monitored by individual amber LOW PRESSURE lamps. The manual describes a warning when the discharge pressure from the relevant boost pump drops below approximately 5 psi. The exact number of annunciators and their locations vary by aircraft variant and pump arrangement. A lamp may reflect a commanded-off pump or an actual pressure deficiency depending on the associated switch and operating conditions; the full fuel-feed chapter is needed for precise pump-by-pump logic.

8.4 Utility and auxiliary hydraulic pressure

The lower panel contains low hydraulic pressure annunciators for the utility and auxiliary hydraulic systems. The source identifies two amber lights for utility hydraulic pressure and two for auxiliary hydraulic pressure in the specified installation. The utility-system low-pressure warning is associated with pressure falling below approximately 1,450 psi. Auxiliary low-pressure warning lights relate to auxiliary hydraulic pressure availability. The exact hydraulic pump and pressure-switch relationships are specified in the hydraulic-system chapters referenced by the TFV.

8.5 Engine vibration

The 707A source identifies red ENGINE VIB warning lights associated with the engine vibration measuring system. The lamps are fitted in the vibration indicator faces and provide a dedicated visual warning when the vibration sensing circuit indicates an excessive condition. The warning-system chapter does not provide a complete vibration threshold calibration.

9. Flight engineer’s auxiliary and overhead panels

9.1 Forward cargo compartment heater

An amber light associated with the FORWARD CARGO COMPARTMENT HEATER indicates a heater condition while the control is selected ON. Depending on variant, the lamp is labelled with caution terminology or indicates heating-system operation. The air-conditioning and cargo heating sections give the details of the associated heater controls.

9.2 Fuel dump chute position

Amber DUMP CHUTE lights identify fuel dump chute positions. The source specifies two lamps for chutes not fully retracted and notes that these lights remain illuminated if the chute has not reached the correct retracted position. This warning is relevant to an aircraft with a fuel jettison system: moving a selector to retract does not guarantee the chute has mechanically reached its stowed position.

9.3 Cargo, galley and entry door warnings

Red door warning lights are provided for cargo, galley and entry doors that are not fully closed and locked. The exact group of lights varies with the 707A, 707B and 707C cabin and cargo configuration. The source names forward main door, aft main door, forward galley door, aft galley door, aft cargo door, and other installation-specific cargo door positions, including a 707C main cargo door. Each lamp is driven by the corresponding door locking arrangement.

A pushbutton switch beneath the door lights allows a DOOR WARN test. The manual provides the general warning-light arrangement here and refers to the individual door system descriptions for sensing and interlock details.

9.4 Equipment cooling airflow

An amber NO AIRFLOW warning is used for equipment cooling. The TFV states that the light illuminates if adequate equipment cooling air is not supplied within the specified period; a period of approximately 10 minutes is described for the particular installation. The warning is an indication of cooling-air failure and should not be treated as a general aircraft ventilation warning.

9.5 Cargo smoke detectors

The manual includes cargo compartment smoke-detection equipment. The relevant installation uses smoke detector warning lamps, a SMOKE DETECTOR control switch, and associated alarm indications. The forward and aft main cargo compartments may use different monitoring arrangements depending on the aircraft. A smoke detector switch may have ON/OFF positions for the corresponding compartment detection circuit. System-specific details are provided in the cargo smoke-detection chapter.

9.6 Passenger oxygen pressure

Three amber lights are associated with passenger oxygen pressure on the overhead panel, indicating oxygen valve or pressure system abnormalities. The labels shown in the source include PASSENGER OXYGEN, VALVE OPEN and a pressure-related indication. The text also differentiates conditions involving oxygen pressure supply and the passenger oxygen valve. These should be linked to the actual oxygen circuit rather than used as generic low-oxygen warnings.

9.7 Q-inlet heat

An amber Q-INLET HEAT light is described for the associated inlet anti-icing or heating arrangement. It illuminates under the specified heating control or fault condition and supplements the Q-inlet heating control circuit.

9.8 Wing anti-ice overheat

An amber WING ANTI-ICE OVERHEAT light illuminates when the wing anti-icing duct or leading-edge protection system develops an overtemperature condition. The source describes temperature-sensitive detection and protective control action, including the control and shutdown consequences of an overheat. The indication is specifically related to the wing anti-ice equipment.

10. Forward electronic panel and pilot’s control stand

10.1 Navigation warning system

On aircraft with the applicable Navigation Warning System, the forward electronic panel contains warning lamps associated with the compass, vertical gyro, localizer and glide slope monitoring functions. They indicate failure or disagreement in a particular navigation or flight reference. The source describes a resettable master navigation warning as well as dedicated failure signals. Because these depend strongly on the avionics installation, a 707 fitted with another navigation or flight director configuration may not display identical annunciations.

10.2 Stabilizer main electric trim

An amber light on the pilot’s control stand warns of main electric stabilizer trim operation under the described malfunction condition. It illuminates when the stabilizer is moved by the main electric trim system under circumstances requiring crew attention. The manual distinguishes warning and test behaviour from ordinary commanded stabilizer trimming.

10.3 Parking brake

An amber PARKING BRAKE light illuminates whenever the parking brake handle is set. This simple indication is particularly important in ground operations, where the same cockpit contains a much wider range of complex master warning and system protection devices.

11. Ground Proximity Warning System (GPWS)

The final section of the Lufthansa document describes a Ground Proximity Warning System and contains an additional temporary revision dated 9 May 1984. This is a later equipment configuration than the original 1960s systems described in earlier portions of the chapter. The GPWS alerts the crew when the aircraft is flown into one or more dangerous flight profiles near terrain. It combines radio altitude, barometric altitude or vertical speed information, aircraft configuration, and navigation-derived information according to the selected mode.

The manual defines five operating modes: Mode 1 — Excessive sink rate; Mode 2 — Excessive terrain closure rate; Mode 3 — Altitude loss after takeoff or go-around; Mode 4 — Unsafe terrain clearance; Mode 5 — Below glide slope. The actual detection uses specific boundary envelopes shown graphically in the source, not just a single altitude or descent-rate number.

11.1 Inputs, arming, priority and flight-deck controls

The GPWS requires valid sensing inputs and radio altitude in its operating range. The source lists its basic inputs as: radio height, barometric altitude rate from the flight-data computer, landing gear position, flap position, and glide slope deviation from the navigation system used by the captain (for Mode 5). Its functional block diagram additionally illustrates attitude warning, altitude/rate, landing-gear, flap and glide-slope interfaces through the associated relays and monitoring circuits. The system uses 28 V DC and 115 V AC, 400 Hz supply lines in the illustrated installation.

The TFV specifies the GPWS as armed with required valid inputs between approximately 50 and 2,500 ft radio height. Its alerting priority is stated explicitly: stall warning takes precedence over ground proximity warning. Among GPWS alerts, Modes 1–4 have priority over Mode 5. If more than one Mode 1–4 alerting condition exists, the warnings can occur at the same time and continue until the relevant conditions are corrected.

The pilot’s panel incorporates a GPWS warning lamp with a PUSH TO TEST function. The source also describes a BELOW G/S amber indicator with a PUSH TO INHIBIT control. On the overhead panel a GPWS INOP indication identifies a fault or unavailable essential input, and a FLAP LOGIC switch permits the landing-flap configuration logic to be modified when the system installation requires it. The manual illustrates both the indication and control locations; it does not state that a GPWS lamp alone diagnoses the failed component.

The BELOW G/S inhibition has a defined purpose: pressing the switch inhibits Mode 5 glide-slope alerting below 1,000 ft radio altitude, as described in the source. This is not a general GPWS OFF switch and does not cancel the terrain warnings of Modes 1–4.

11.2 Mode 1 — Excessive sink rate

Mode 1 monitors descent rate in relation to radio altitude and generates an alert when the aircraft descends too quickly for its height above the ground. The prescribed aural warning is “WHOOP WHOOP PULL UP”, accompanied by a GPWS visual warning. The source explains that the sink-rate boundary depends on altitude and is represented by a sloping warning envelope rather than a fixed 500, 1,000, or 2,000 ft/min limit.

Mode 1 operates within the relevant lower-altitude radio-height envelope up to approximately 2,500 ft. Its chart shows that the permissible sink rate generally increases with height. The manual also describes flight-configuration differences and the warning’s relationship to aircraft descent through the envelope. At very low heights the protection is restricted or inhibited according to the mode logic, preventing inappropriate operation in the final stages of a normal landing.

For a software reconstruction, Mode 1 therefore requires a calibrated two-dimensional boundary, using radio altitude and barometric descent rate. A simple test such as “descent rate over 2,000 ft/min” would not accurately reproduce the Lufthansa equipment.

11.3 Mode 2 — Excessive terrain closure rate

Mode 2 detects dangerously rapid closing movement toward the terrain. The manual explicitly explains that this mode offers protection during approach, as the aircraft descends toward increasing ground elevation, and in situations where the aircraft flies nearly level toward rising terrain. It relies on terrain closure rate derived from radio altitude change rather than merely the vertical speed indicated by a conventional barometric VSI.

The audible annunciation is again “WHOOP WHOOP PULL UP” with the GPWS visual warning. Mode 2 is shown as having distinct warning boundaries for landing flap and non-landing flap configurations. The source chart indicates a lower-height envelope for landing flap configuration and a more extensive envelope for non-landing configuration, rising to approximately 1,800 ft radio height under some conditions.

These configuration-sensitive envelopes matter because a terrain closure rate that is acceptable in a properly configured landing may be dangerous when the aircraft is not configured to land. The mode is not a forward-looking terrain database system; it responds to measured changes in radio altitude and the associated flight parameters.

11.4 Mode 3 — Altitude loss after takeoff or go-around

Mode 3 protects against an abnormal loss of altitude after takeoff or during a go-around. The manual describes the mode as remaining active below approximately 700 ft radio height in the appropriate takeoff or go-around operating condition. It monitors loss of barometric altitude after the aircraft has established the reference height used by the mode.

The Mode 3 warning is “WHOOP WHOOP PULL UP”, with the GPWS lamp illuminated. The supplied graph indicates that the tolerated altitude loss grows with radio altitude; the envelope reaches approximately 70 ft of loss at the upper part of its stated range. A threshold of 70 ft is not therefore a universal trigger at every height. The logic is designed to distinguish normal variations immediately after liftoff from a potentially dangerous subsequent descent.

Mode 3 requires a stored reference to the altitude previously reached, along with an understanding of the current takeoff or go-around phase. Simply comparing present vertical speed with zero would not reproduce it.

11.5 Mode 4 — Unsafe terrain clearance

Mode 4 detects inadequate terrain clearance with an inappropriate landing configuration. According to the manual it is divided into three submodes. The two operational situations specifically described are gear not in landing position and flaps not in landing position; warning envelopes vary with aircraft speed or descent and configuration.

The primary aural message is “WHOOP WHOOP PULL UP”, along with the GPWS visual warning. The source includes two charts: one showing a constant radio-height region of approximately 500 ft for a warning condition, and another showing a boundary related to barometric sink rate with an initial low-altitude band around 200 ft and a rising limit to about 500 ft. These plots correspond to different submode conditions; they should not be combined into one generic terrain-clearance threshold.

A gear-configuration warning below a certain altitude is not equivalent to the older landing gear horn. The landing-gear horn responds to gear, flap, throttle and switch configurations. GPWS Mode 4 combines its configuration information with radio height and additional flight parameters to recognize an unsafe terrain proximity profile.

11.6 Mode 5 — Below glide slope

Mode 5 warns of an excessive deviation below an ILS glide slope. The manual states that the mode is automatically armed when the pilot selects an appropriate ILS frequency. When the measured glide-slope error exceeds the programmed threshold, the system emits the audible message “GLIDE SLOPE”. The repetition rate increases as the deviation below glide slope becomes greater.

The source describes Mode 5 as active below approximately 1,000 ft radio height. Its warning envelope is based on glide-slope deviation in dots, with a lower threshold at the higher part of the altitude range and a larger deviation permitted close to the ground. The associated chart begins at about 1.3 dots below glide slope and illustrates increasing permitted deviation at lower radio heights. The amber BELOW G/S light is displayed in the cockpit while the mode is alerting.

Mode 5 has lower priority than the other four GPWS modes. This is essential during an unsafe approach: a severe sink-rate or terrain closure warning should take precedence over repeated glide-slope deviation calls. The BELOW G/S PUSH TO INHIBIT control permits the crew to inhibit Mode 5 beneath the specified 1,000-ft height, without disabling the other GPWS functions.

11.7 Automatic changes in GPWS operating modes

The temporary revision describes automatic logic transitions linked to aircraft altitude and flight phase. For example, automatic mode switching occurs when an aircraft climbs through a threshold around 1,000 ft radio altitude, or after descending below a specified radio-height value when landing configuration is established. The described transition logic includes conditions around 200 ft radio height and the landing-gear position. The exact changeover is a state-dependent sequence rather than a simple five-position selector switch.

The source also describes conditions for automatic deactivation of Mode 5, including invalid glide-slope signals and specific aircraft operating states, and conditions for automatic reactivation after an inhibit when the aircraft subsequently climbs through the reset radio-height threshold. These details should be transcribed into a state machine if a faithful simulation of the 1984 GPWS installation is intended.

11.8 GPWS indications and installation details

The GPWS schematic shows the monitoring and control unit, warning lamps, navigation inputs, power supplies and the cockpit speaker connections. The diagram is valuable because it establishes which original aircraft signals feed the computer, even where the source text does not detail the exact electronic sensing method inside the unit.

12. Electrical power, annunciation and reset principles

The warning installation reflects the Boeing 707’s mixed electrical architecture. Warning circuits use 28 V DC from selected buses, including essential-bus services; the GPWS section also depicts a 115 V AC 400 Hz supply. The relevant warning is therefore affected not only by the physical fault and switch position but also by which source and bus are energized. The TFV repeatedly identifies power feeds such as battery and essential power, and associates lights with relay operation, pressure switches, microswitches, thermostats and electronic monitoring units.

Four principles are worth retaining when interpreting the system. First, a warning may be latched, flashing, or intermittent rather than directly representing the instantaneous position of a switch. Second, a reset or cutout button often silences or acknowledges the crew alert without cancelling the original fault. Third, some panel lamps represent normal system status, such as a heater valve position or parking brake set, despite being included in the warning chapter. Fourth, warning validity is dependent on power supply and sensing equipment; a dark warning light cannot always be interpreted as proof that the monitored system is healthy.

13. Historical interpretation and simulator implementation

For a systems-level recreation, the best approach is to model the warning circuits from their underlying conditions rather than driving the lights from generic X-Plane annunciators. Landing gear door locks, fire detectors, generator oil-pressure switches, bus tie positions, fuel boost-pump discharge pressure, hydraulic pressure, and aircraft configuration can each be represented as explicit states. A discrete warning layer can then evaluate these states and drive the corresponding light, bell, horn, chime or GPWS output.

The horn should be treated as a multi-source device with separate continuous and interrupted signals. The fire bell requires a distinguishable cutout function. The master warning and navigation warning systems require their own reset/acknowledgement logic. A dedicated annunciator inventory should retain the actual 707A/707B/707C variant flags described in the manual, so that the simulation does not accidentally display lights from mutually different aircraft configurations.

For GPWS, the five modes should be implemented as a separate computer that consumes radio altitude, barometric descent rate or altitude history, radio-altitude closure rate, gear status, flap state and captain-side glide-slope deviation. Each mode needs its own envelope and inhibit logic, with a priority arbiter for aural outputs. A modern X-Plane terrain alert, although potentially similar in effect, is not a suitable substitute for the historical envelope- and configuration-based GPWS described here.

Not all numerical details required for a fully exact simulation are available as machine-readable parameters in the supplied chapter. Some GPWS boundaries are provided only as plotted graphs; certain warning circuits refer to separate Lufthansa wiring diagrams. A faithful implementation therefore requires either digitizing those graphs and consulting their referenced system chapters or transparently labelling an approximate threshold. The numbers in this article are included where they are legible and supported by the manual; they should not be treated as certified software-calibration data.

14. Source reference, aircraft variants and limitations

Primary source: Lufthansa Technische Flugdienstvorschrift, TFV Boeing 707, Chapter 10.20, Warning Systems, supplied 31-page document. Sections 10.20.1–10.20.4 describe the warning installation and its illustrations; Section 10.20.5 covers the Ground Proximity Warning System. Visible document revision dates include 20 December 1965, later pages dated 1973–1978, and Temporary Revision 84-1/84-2 dated 9 May 1984.

The text is a structured English technical summary, not a reproduction of the Lufthansa maintenance instructions, wiring specifications, or approved flight procedures. Variant differences and later modifications are retained as far as the source identifies them. For precise relay-level logic, switch thresholds, part numbers, or airworthiness purposes, the applicable original system manuals and aircraft configuration records remain authoritative.