Boeing 787 Landing Gear and Electric Brakes

The Boeing 787 rests on a conventional retractable tricycle landing gear, two nose wheels and two four-wheel main bogies, but it stops differently from the 777: its wheel brakes are electric. Gear extension and retraction and nose wheel steering remain hydraulic, and the steering is commanded by wire from the tillers and the rudder pedals. The 787-10 adds a semi-levered main gear of its own.

What the hydraulic systems still power, including the isolation systems for reserve steering and alternate gear extension, is covered in Boeing 787 hydraulic system. This article draws on the EASA type-certificate data sheet (TCDS), the FAA Master Minimum Equipment List (MMEL), the FAA Flight Standardization Board (FSB) reports, Boeing's airport planning document and Boeing's 2007 AERO article. The A220's arrangement is described in A220 landing gear, brakes and steering.

Landing gear layout

For all three models the TCDS lists two nose wheels and eight main wheels. Each main gear is a twin tandem bogie of four wheels and, unlike the 777's, has no aft axle steering: the 787 is steered through the nose gear alone. The tyre sizes are given in the TCDS and the pressures in Boeing's airport planning document:

Air/ground sensing uses tilt sensors and strut compression sensors on each main gear, eight air/ground sensors in all, and one of each per main gear may be inoperative for dispatch. The MMEL also lists a proximity sensing system with six proximity sensor data concentrators. A tail strike detection system with two channels generates a TAIL STRIKE caution message; with both channels inoperative, the tail must be inspected for damage before each departure and the caution message is disabled.

The 787-10 is the only model with semi-levered main landing gear. The FSB differences tables list it as added on the 787-10, with a difference in the exterior preflight, at level A. The MMEL marks its items "(787-10)": two semi-lever gear systems, a semi-lever gear lock system with left and right lockup valve control systems, a hydraulic isolation valve, hydraulic pressure sensors, gas pressure and temperature sensors and a telescoping piston. With the lock system inoperative, the lockup valve control systems must be deactivated, the semi-lever gear tilt functions checked once each flight day and appropriate performance adjustments applied. The 787-10 keeps the same two nose and eight main wheels as the other models.

Normal extension and retraction

The gear is moved hydraulically: landing gear actuation is one of the loads of the three hydraulic systems in Boeing's description. The landing gear control relays and solenoids are arranged in two system channels, and the MMEL allows relays or solenoids on one channel to be inoperative only if the opposite channel works. It also lists a landing gear lever lock solenoid, six landing gear control lever switches, main and nose gear control sensors, landing gear door safety valves with a STOW position, and nose and main gear bypass/auto-off valves.

For indication, the MMEL lists a landing gear position indication system, with two installed, landing gear door close position and main gear uplock position sensors, and a landing gear synoptic display. It also refers to a gear extend speed placard; like every airspeed limit other than VMO and MMO, the gear speeds are left by the TCDS to the flight manual (see Boeing 787 family and flight deck). From the 777-300ER, the FSB rates the main gear, nose wheel steering tiller and brake differences at level A.

With the landing gear actuation system inoperative, the 787 may still be dispatched with the gear secured down under the flight manual's Landing Gear Extended Appendix, gear down dispatch being one of the approved kinds of operation in the TCDS. The same appendix applies with the semi-lever gear system, the landing gear position indication or a bypass/auto-off valve inoperative. Principles are in landing gear retraction and extension and landing gear indication and configuration warnings.

Alternate extension

A separate landing gear alternate extend system is listed in the MMEL. Its dispatch relief requires either that the landing gear doors be verified to open using the alternate extend system, with the landing gear control relays, solenoids, sensors and lever switches all working, or that the gear be secured down, the alternate extend system deactivated and the aircraft dispatched under the Landing Gear Extended Appendix. A ground door release control system is listed alongside it, with door open control switches and a ground door release closing function.

The hydraulic article explains that the MMEL ties an alternate gear extension isolation system to the centre hydraulic system.

Electric brakes

The original FSB report lists electric brakes among the differences from the 777, with brake system operation functionally equivalent, and the FAA's 777 report lists hydraulic brakes as a difference when going the other way, from the 787-8 to the 777-300ER. Each of the eight main wheels has a wheel brake with four electric brake actuators: the MMEL lists 32 electric brake actuator systems, of which one per wheel may be inoperative, leaving 24 for dispatch. The EASA certification tables name a brake system control unit and an electric brake actuator controller. The brakes are one more job the 787 gives to electricity, alongside the cabin air compressors (air conditioning and pressurisation) and the wing ice protection (ice and rain protection).

One indication changes with the technology: where the 777 shows a brake accumulator, the 787 shows a battery indication. The nose gear has two spin brakes but no wheel brakes, all eight wheel brake systems being on the main gear, and the MMEL lists three brake status lights on the nose gear.

The MMEL's brake provisos concern the take-off. With one wheel brake per truck deactivated, the electric brake actuators on the other wheels of that truck must work, flaps 20 may not be used for take-off, performance adjustments apply and the gear must stay down for two minutes after take-off before it is retracted, unless an engine fails after V1. The same flaps 20 and two-minute conditions apply with a nose gear spin brake inoperative.

A brake temperature monitoring system (BTMS) is fitted; with it inoperative, the flight manual's maximum quick turnaround weight limitations must be observed. A tyre pressure indication system (TPIS) covers both the nose and the main wheels. Principles are in wheel brakes and brake energy and tyre speed limits.

Antiskid and autobrake

Antiskid control is per wheel: the MMEL lists eight antiskid control systems, one for each main wheel. One may be inoperative on one main gear wheel per truck, with that wheel's brake deactivated and the same flaps 20, performance and two-minute gear retraction conditions as for a deactivated brake.

The 787 also has an autobrake system, operated with an autobrake selector. With the system inoperative, the MMEL allows dispatch provided the selector remains OFF. The principles are in anti-skid and autobrake.

Nosewheel steering

The original FSB report describes the nose wheel steering tiller as steer by wire and functionally equivalent to the 777's. Each pilot has a tiller, and the rudder pedals also steer, the steering itself being one of the hydraulic loads. The MMEL lists:

• two nose wheel steering channels, one of which may be inoperative;
• three rudder pedal steering channels, two of which are required;
• six tiller channels, of which one in each tiller may be inoperative;
• two nose wheel steering rudder pedal disconnect switches, used to disconnect nose wheel steering from the pedals for rudder sweeps.

One tiller may be inoperative if the other works and alternate procedures are used. The original FSB report also lists a nose gear cutout switch among the tiller differences from the 777.

Boeing's turning radius tables run up to a maximum nose gear steering angle of 70 degrees on all three models, while the clearance radii use an effective turning angle of 65 degrees, or 64 degrees on the 787-10. The TCDS approves towbarless towing. Principles are in nose-wheel steering.

Limits and indications

The dispatch conditions in the MMEL summarise what the gear and brakes depend on:

The wheel wells have their own fire detection, twelve temperature detectors, described in Boeing 787 APU and fire protection.

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.