Heritage Concorde

Concorde’s development phase raise another issue concerning her landing gear. Its large delta-wing requires a high angle of attack, which leads to a pitch angle on touchdown of 11 degrees. This requires the landing gear to be unusually strong due to the unusual loadings which it would have to cope with. This angle of attack caused another problem, it put the rearmost part of the engines nearest to the ground, this required the legs to be long enough to stop them touching. So the increased weight loading and length required a major redesign of the landing gear for Concorde.

As part of the 2001 modification following the 2000 Paris crash. The original standard aviation design nylon bias ply tyres on all eight main wheels were replaced with a new puncture resistant lighter weight tyre, the NZG Near Zero Growth tyre developed by Michelin Aviation Products. The Michelin tyres are rim-to-rim reinforced radial tyres that are resistant to incision. If severe damage should occur to the tyre it is designed to fragment into very small pieces too small to result in rupturing a tank. Expansion of the tyre under pressure and consequential elongation of the rubber tread is minimised by reinforced belts and sidewalls. The electrical wiring in the undercarriage was encased in steel braiding as part of the modifications carried out during 2001.

Concorde is equipped with a hydraulically operated retractable tricycle landing gear layout, with a nose gear consisting of two wheels and 2 main gears consisting of 4 wheels each. Separate from this configuration is a tail bumper gear consisting of 2 wheels, that is fitted to prevent any damage to the fuselage and engine nacelles, should the aircraft suffer too high an angle of attack during take-off or landing.

One interesting note about the main landing gear is that if both were to just swing up to be stowed away they would hit each other and jam. The combined length of both undercarriages is greater than the distance between both undercarriage roots. This problem required that the undercarriage be first retracted vertically and then swung inwards to be tucked in the wing and fuselage belly.

Concorde maximum permissible weight for the start of any ground manoeuvre is 186,880kgs and for the start of take-off 185,070kgs. At the point of rotation, the main landing gear, which is literally the centre of rotation, is loaded up at 195kts as the rear end squats and the frontend is hauled off the runway. The maximum landing weight is 111,130kgs, though in 1981 landing at up to 130,000kgs was authorised. Normal approach speed (Vref) varies with the aircraft weight from 150kts to 162kts at maximum landing weight, however a Vref+7kts became the standard in 1979. At 130,000kgs, Vref was up to 175kts while on the day something quite unpleasant could happen and Concorde had to make an immediate return landing, the maximum weight Vfef would be 207kts and 238mph. These are just some of the numbers that the French landing gear manufactures had to contend with.

The landing gear is a spring/damper unit providing suspension and damping by an oil/gas medium (oleo/pneumatic). The oil is of the hydraulic system, the familiar pink DTD 585, while the gas has been changed from compressed air to nitrogen. First and foremost, the landing gear is a shock absorber, its job, to absorb the dissipate kinetic energy at landing.

Main landing gear

 

Concorde’s main landing gear comprises of two sets of four-wheel bogies.  The main landing gear was designed by Messier-Dowty and has to shorten during the retraction process, as it would otherwise not be able to fit into the bays in the wing roots after take-off. It is also fitted with a spray guard to prevent water from wet runways being flung-up and sucked into the engine air intakes and also fitted with Oleo-pneumatic shock-absorbers.

During the development phase of Concorde’s life, the main landing gear location was a real headache. At the only logical position, the legs would have collided as they were retracted upwards and inwards, they were basically too long.  So they were shortened, so whenever the gear was down and locked they were at their full length, but during the retraction process, a mechanical linkage gradually and completely pulled the  oleo inside the barrel, a simple, yet effective and trouble-free solution.

Main landing gear facts

Number of wheels on each bogie – 4

Direction of retraction -Inwards

Tyre size – 47X15.75-22

Tyre type – Michelin NZG

Tyre pressure – 232PSI

Brakes – 4 X Dunlop Carbon Fibre with SNECMA (Hispano) SPAD anti-skid units

The manufacturer – Messier-Dowty

The emergency Operation of the main landing gear – A, Hydraulically lowered by standby system. B, Mechanical release and freefall to lock

Normal operation – Hydraulic system (Green)

Nose Landing gear

 

There are many reasons why Concorde’s landing gear differs from that of other passenger aircraft. One of these is that the steerable twin wheeled nose unit is situated behind the fight deck, and makes taxing very different to that of other aircrafts. The landing gear location and dimensions, although critical have been in fact a compromise. The nose landing gear is in a far from idea location, it is 40 ft aft of the nose; think about having to turn on a taxiway, T-junction or following the centre line into an arrival gate between two other aircraft with the steering that far behind you.

Another difference is that it retracts forward up into the belly of the fuselage, this was because of the need to plan for the free-fall mechanism; The nose landing gear was designed and built by Hispano.

The nose landing gear is used for steering Concorde on the ground and unlike most of Concorde, the steering is entirely conventional, it is electrically signalled from either pilot’s stations on the flight deck, and powered by the main hydraulics with auto-change to standby hydraulics. The steering control handles will operate the wheels + or – 60 degrees;  flight control rudder pedals will steer + or – 10 degrees while Concorde is on the ground only.

Should the hydraulics fail, and then the standby system  fail too, there is a free-fall mechanism for the nose landing gear which is located under the passenger cabin floor.

Nose landing gear facts

Number of wheel on the bogie – 2

Direction of retraction – Forwards

Tyre size – 31X10.75-14

Tyre type – Dunlop (British Airways) Goodyear (Air France)

Tyre pressure – 190PSI

Brakes – None fitted

Steering – Electrically signalled and hydraulically controlled

The manufacturer – Hispano

The emergency Operation of the main landing gear – A, Hydraulically lowered by standby system. B, Mechanical release and freefall to lock

Normal operation – Hydraulic system (Green)

 

NOSEWHEEL STEERING OPERATION

 

 

Tail bumper wheel

 

The retractable tail bumper wheel was designed to protect the rear of Concorde during takeoff and landing, since she needed a high angle of attack to optimise lift from her delta wing.

The prototypes and pre-production Concordes had a sort of tail bumper or skid, and this type of tail landing gear was used as a replacement. Should the tail wheels touch the ground on landing, and its shock absorber compress, then the first point of contact is actullally the lower part of the secondary nozzles as they translate to the reverse thrust position.    In fact you can see a repair to a secondary nozzle on Concorde G-BOAF at Filton. There is no free-fall release for the tail landing gear.

Tail bumper wheel facts

Number of wheel on the bogie – 2

Direction of retraction – Rearwards

Brakes – None fitted

The emergency Operation of the Tail bumper – None

Normal operation – Hydraulic system (Green)

Concorde’s Brakes

 

Dunlop and Concorde changed the design of big aircraft brakes forever. Still powered from a main hydraulic system with emergency operation supplied from standby hydraulics. Still a multi-disc unit using five rotors keyed to the wheel and six stators keyed to the axle. But now these discs are structural carbon fibre – solid discs of carbon fibre.

Most of the test flights of the Concorde program were conducted using the conventional steel brakes, but during this time Dunlop were experimenting with carbon fibre and of course the production methods needed. The first production unit was cleared for trials in 1972 and fitted to a BOAC VC10, in 1974 they became the standard, for Concorde to fit and the industry followed. These brakes with the assistance of reverse thrust, had to be able to bring 184 tonnes of Concorde from 165kts down to a standstill. In engineering terms, convert the aircraft Kinetic Energy (KE=1.5 mv sq) into heat and store that heat safely until it is dissipated either by natural cooling or by forced ventilation as with the brake fans fitted to Concorde.

Due to a relatively high average takeoff speed of 250 mph (400 km/h), Concorde needed good brakes. Concorde used an anti-lock braking system,which stop the wheels from locking when fully applied, allowing greater deceleration and control during braking, particularly in wet conditions. The brakes, developed by Dunlop,were carbon-based and could bring Concorde, weighing up to 185 tons (188 tonnes)and travelling at 190 mph (305 km/h), to a stop from an aborted takeoff within one mile (1600 m). This braking manoeuvre brought the brakes to temperatures of 300 °C to 500 °C, requiring several hours for cooling using the fans.

Concorde’s carbon discs brakes are controlled by a hinged toe-plate attached to the pilots’ rudder pedals. They are powered hydraulically and controlled by electrical signals from the toe-plates. An advance anti-skid system modulates the braking pressures applied by the pilot. Concorde uses signals from its nose wheels (which is not braked and therefore does not skid) to vary the reference value so that the modulation of the brake pressure always happens close to the optimum at all speeds.

A brake control lever is located at the co-pilot’s side of the centre console: fully forward for normal brakes, first position rearwards selects emergency brakes – no electronics, but  with an accumulator charged to 3,000ils/sq in; depress the lever’s button and move to second rearwards position for parking brake, operating via emergency system.

Normal brakes and anti-skid systems are supplied by the Green hydraulic system. In the event of a Green system pressure loss there is an automatic changeover to the Yellow hydraulic system provided YELLOW-GREEN has been selected on the Servo Control yellow rotary selector.

A brakes accumulator charged by the Yellow hydraulic system supplies the emergency and parking brakes system only.

 

Landing Gear Retraction and Extension

 

NORMAL OPERATION

 

Retraction and normal extension is electrically signaled and hydraulically actuated. The landing gear up selection initiates a sequence of actions. If the up-locks are open and ready, the doors for the three principal gears are opened, then if nose wheel steering is centered and main bogies level, hydraulic pressure opens the down-locks, unlocks the shortening mechanism and retracts the gear. When up-locks are engaged, the doors will close. Extension is far simpler: its checks that the down-locks are open then sequences doors open, gear down and door close. Tail gear basically followers the others, but is not involved in sequencing. Maximum speed for gear operation is 270kts; it takes about 12 second to retract.

When the landing gear lever on the flight deck is in the NAUTRAL position, both the electrical control and hydraulic supply are shut off leaving the doors held by the mechanical locks, thus preventing inadvertent land gear extension through control or actuator failure. During the landing gear retraction the main and nose wheels are automatically braked and released.

STANDBY OPERATION

 

In the event of hydraulic malfunction there is a standby lowering system. It is accomplished hydro-mechanically and by manual sequencing and timing, by means of a separate control lever on the Centre Pedestal Instrument Panel on the flight deck, the lever is independently powered by the Yellow hydraulic system.

FREE FALL OPERATION

 

Certain combinations of problems would plan to preserve the hydraulic effort exclusively for the flight controls. In these cases the landing gear would be lowered by free-fall mechanisms. This involves a visit to the passenger cabin and opening a floor panel. Free-fall controls are located immediately above the nose gear and the main-gear up-locks. For the nose-gear a rotary control first isolates hydraulics and vents the retraction jack to atmosphere, then via a screw jack mechanism opens the doors up-locks and the gear up-locks. The gear falls into the doors pushing them open, the airstream catches the gear and pushes it into the down-lock.

Under the rear cabin floor a rotary control does the similar job on the main landing gear hydraulics, but this time a cranked lever is placed into a socket and, from a kneeling position with body braced and trying not to grimace it is heaved forcefully to starboard to open the up-locks.

In both cases a change in slipstream noise will indicate the measure of success. A practiced hand will achieve ‘3 green’ within 2 minutes and 45 seconds. At this point the tail gear would remain in the up position.

To assist in locating the free fall controls, a starboard hat rack in both forward and rear cabins is identified with 1 red and 1 green disc at a position adjacent to each control panel.