Boeing 707 Flight Controls

The Boeing 707 uses a flight-control architecture that is very different from the fully powered systems found on many later jet transports. Much of the pilot’s input is transmitted mechanically through cables, quadrants, rods and tabs, while aerodynamic balance panels and control tabs reduce the forces required to move the main control surfaces. Hydraulic power is used where high loads make it necessary—most notably for the spoilers, the flap drives and the normal rudder power unit.

System philosophy

The basic primary controls are the ailerons, elevators, rudder and spoilers. The secondary controls are the wing-flap system and trim systems. The Lufthansa Technical School material describes the inboard ailerons and elevators as predominantly aerodynamic controls: pilot movement positions a control tab, and the tab’s aerodynamic force drives the larger control surface. Balance panels on the leading side of the surface use the pressure difference between the upper and lower sides to assist movement and holding force. This arrangement reduces the need for continuously powered hydraulic actuators on the ailerons and elevators.

The training binder covers several 707 standards and repeatedly distinguishes the early 707A from the later 707B/C arrangement. The largest differences described in Chapter 27 concern the flap and leading-edge systems. The 707A has only four leading-edge flaps and true split-type fillet flaps, whereas the 707B/C uses a much more extensive leading-edge arrangement with inboard leading-edge flaps and slats farther outboard. The 707B/C inboard flap drive is also more powerful and operates the leading-edge control mechanism. Where the arrangements differ, this overview follows the 707B/C description because it represents the later configuration while preserving the manual’s explicit distinctions.

The rudder is different. In normal operation it is hydraulically powered from the Auxiliary hydraulic system. If hydraulic rudder power is lost, the mechanical linkage changes function: the rudder control tab is released from its normal anti-balance role and becomes a Flettner-type control tab that can move the rudder aerodynamically. The spoilers are also hydraulic and form an integral part of both roll control and the speed-brake system.

Lateral control: inboard and outboard ailerons

Control-wheel rotation is transferred through drums and cable systems to the inboard aileron quadrants. From each quadrant, separate pushrods operate the inboard aileron control tab and the spoiler control valves. The inboard aileron itself is not normally driven by a hydraulic actuator. Instead, the control tab creates an aerodynamic moment that moves the aileron, while two balance panels assist the motion.

At low aerodynamic loads, such as on the ground or at low speed, the tab alone may not generate enough force. The linkage therefore includes a mechanical pickup: when the control tab reaches its travel limit, the mechanism can directly move the inboard aileron. The inboard aileron snubbers serve two functions. They protect the structure against gust movement and limit aileron angular velocity to approximately 50 degrees per second. The training material states that they are intended to prevent damage from gusts up to 70 mph.

The outboard ailerons are available only when the flaps are partly or fully extended. A flap-driven lockout mechanism changes the geometry of the outboard-aileron linkage and connects the outboard ailerons to the lateral-control system as the flaps extend. With the flaps retracted, the outboard ailerons remain locked in neutral. Their balance tabs move opposite to the aileron and reduce the control force.

Spoilers and speed brakes

There are four spoiler panels on each wing. For roll assistance, spoiler pairs rise on the wing where the inboard aileron moves upward. The inboard spoiler pair is supplied by the Auxiliary hydraulic system and the outboard pair by the Utility system. Mechanical linkage from the inboard aileron quadrant moves the spoiler control valves. A follow-up mechanism returns each valve to neutral when the commanded spoiler angle has been reached.

Electrical bypass valves allow the spoiler hydraulic supply to be isolated. In bypass, pressure is removed and the spoiler lines are connected to return; aerodynamic pressure then forces an extended spoiler back toward the wing.

The cockpit speed-brake lever provides symmetric spoiler deployment from 0 to 60 degrees, with a 30-degree detent. Roll commands can still modify the spoiler positions around the selected speed-brake position. The system is also naturally load-limited. The manual gives an example of approximately 20 degrees maximum spoiler angle at 300 knots and about 11 degrees at 350 knots for a 3000 psi limit condition. If aerodynamic loading pushes the spoiler back, hydraulic oil is displaced until the relevant main relief valve opens at approximately 3500 psi.

Wing flaps and leading-edge devices

The main wing flaps are Fowler-type surfaces divided into inboard and outboard groups. Two independent drive systems are used: the inboard flaps are powered from a unit in the left main-wheel well and the outboard flaps from a unit in the right main-wheel well. Under normal conditions each system uses a multi-piston hydraulic motor. Torque tubes, gearboxes and ball-screw mechanisms convert motor rotation into flap travel. Follow-up gearing stops the system when the actual flap position reaches the position selected by the cockpit lever.

The flap lever has the positions UP, 20, 30, 40, 50 and DOWN. The cockpit overview associates 20 degrees with 220 KIAS, 30 with 210 KIAS, 40 with 200 KIAS and 50 with 185 KIAS. The hydraulic flap-drive units require about 6 gal/min for the outboard system and 9 gal/min for the inboard system. Normal extension or retraction of the main and fillet flaps takes approximately 30 seconds.

For the 707B/C arrangement, the leading-edge system consists of leading-edge flaps inboard of the inboard engines and slats farther outboard. The leading-edge control valve is driven from the inboard flap drive. When the main flaps pass approximately 9.5 degrees during extension, the leading-edge devices are commanded fully out. During retraction they remain extended until the main flaps return to about 6 degrees. The manual gives a leading-edge deployment time of roughly five to eight seconds.

If hydraulic flap power is lost, separate 115-volt three-phase electric motors can drive the inboard and outboard systems. The emergency drive is much slower: approximately 2.5 minutes for the outboard flaps and 4.5 minutes for the inboard flaps to travel between 0 and 50 degrees. A torque limiter disconnects the emergency drive at about 1300 in·lbf and reduces the load to about 100 in·lbf after release. The manual also requires approximately ten seconds between reversals of emergency-drive direction.

Elevator control

The horizontal stabilizer carries two separate elevators. Each elevator has an outer control tab and an inner stabilizer-actuated tab. Pilot fore-and-aft column movement is transmitted by cables to the aft elevator quadrant and then through rods and torque tubes to the outer control tabs. At normal flight speed the control tab moves first, producing the aerodynamic force that drives the elevator. At low speed or on the ground, a mechanical pickup at the end of tab travel directly moves the elevator.

Six divided balance-panel sections are installed ahead of each elevator. Seals maintain the useful pressure difference across the panels so that they assist the elevator in moving and holding position. A hydraulic snubber is installed between each elevator and the stabilizer. According to the training material, it limits elevator travel to approximately 23 degrees up and 13 degrees down and provides protection against gusts up to 70 mph. Internal metering also cushions the motion near the travel limits.

The inner elevator tabs are linked to stabilizer position. Their first task is to keep the elevator approximately aligned with the stabilizer as stabilizer incidence changes. Their second task is to modify column force within a defined trim range, adding a deliberate force characteristic rather than simply allowing the elevator to float freely behind the stabilizer.

Horizontal stabilizer trim

Longitudinal trim is obtained by changing the incidence of the complete horizontal stabilizer through a ball-bearing jackscrew. The same mechanical system can be driven in three ways: manually from the cockpit trim wheels, electrically from the control-wheel trim switches, or automatically by the autopilot/Mach-trim servo.

Electric trim uses a 115/200 V AC three-phase motor and two 28 V DC magnetic clutches. Selecting NOSE UP or NOSE DOWN energizes the motor and the appropriate clutch, which determines jackscrew direction. The cockpit overview gives an electric trim rate of 0.5 trim units per second. A torque limiter protects the drive. The training text states that it releases at approximately 1500 in·lbf and then allows only about 500 in·lbf.

Primary and auxiliary brakes prevent aerodynamic loads from back-driving the stabilizer. In addition, the aircraft has a trim-brake mechanism intended to stop a runaway trim condition when the pilot moves the elevator control column against the unwanted stabilizer motion. Electrical cutout switches remove power from the electric and automatic trim circuits, while manual trim remains mechanically connected through the trim wheels, chain and cable system.

Rudder, rudder feel and yaw damper

Pedal movement is carried by cables to an aft quadrant in the vertical tail. With hydraulic power available, the linkage displaces the control valve in the rudder power unit. Auxiliary-system pressure then moves the power-unit piston and the rudder. Follow-up linkage returns the valve toward neutral when the commanded rudder position has been reached.

The detailed rudder chapter states that the power unit receives 3000 psi below 250 knots and that pressure is reduced to 2250 psi above 250 knots to limit structural loads. A pressure-control valve and pressure reducer perform the change. The same chapter explains that the rudder-feel unit adds pedal force as a function of rudder displacement and dynamic pressure. Its Q-spring is effectively removed from the feel system below 130 knots and begins contributing above 130 knots.

The yaw damper enters the hydraulic summing mechanism without moving the pilot’s pedals. Its electric command is converted into hydraulic differential pressure by the transfer valve, which then shifts the power-unit control valves. The manual limits yaw-damper rudder authority to approximately ±4 degrees. The pilot can override the yaw-damper command through normal pedal input.

If hydraulic rudder power is lost, the power unit bypasses internally and the hydraulic tab lock releases. Pedal movement then drives the rudder control tab mechanically. The tab produces an aerodynamic moment that moves the rudder, assisted by the rudder balance panels. In powered operation the same tab acts as an anti-balance tab, increasing the desired control-force characteristic.

What makes the 707 system distinctive

The defining feature of the Boeing 707 control system is not simply that it is “manual” or “hydraulic.” It is a hybrid architecture in which mechanical linkages, aerodynamic tabs, pressure-balanced surfaces and selected hydraulic power systems are deliberately combined. Roll and pitch rely heavily on aerodynamic servo action; the spoilers add hydraulic roll authority; the rudder is normally hydraulically boosted but retains a tab-driven fallback; the flaps use independent hydraulic power units with slow electric emergency drives; and stabilizer trim combines manual, electric and automatic inputs on one mechanically interconnected jackscrew system.

That architecture gives the 707 a characteristic relationship between airspeed, control force and control-surface response. The system is full of mechanical follow-up loops, lockouts, pressure reductions and aerodynamic load limits. Those details are essential to understanding why the aircraft behaves differently from later jets with fully powered, irreversible controls.

Source: Lufthansa Technical School, Boeing 707, Chapter 27 “Steuerungsanlagen”, training edition 10/71. The detailed rudder chapter specifies 3000 psi below 250 kt and 2250 psi above 250 kt. An earlier cockpit-overview page in the same binder shows a different 245 kt / 1000 psi indication; that discrepancy should be resolved against the exact aircraft standard before using the value for engineering or simulator certification.