Several aviation incidents and accidents have occurred in which the control surfaces of the aircraft became disabled, often due to failure of hydraulic systems or the flight control system. Other incidents have occurred where controls were not functioning correctly prior to take-off, either due to maintenance or pilot error. Aircraft are not designed to be flown in such circumstances, however a small number of pilots have had some success in controlling aircraft with disabled controls.
Control techniques
A basic means of controlling an aircraft with disabled flight controls, discovered through crew experience, is by making use of the position of the engines. If the engines are mounted under the centre of gravity, as is the case in most passenger jets, then increasing the thrust will raise the nose, while decreasing the thrust will lower it. This control method may call for control inputs that go against the pilot's instinct: when the aircraft is in a dive, adding thrust will raise the nose and vice versa.
Additionally, asymmetrical thrust has been used for directional control: if the left engine is idled and power is increased on the right side this will result in a yaw to the left, and vice versa. If throttle settings allow the throttles to be shifted without affecting the total amount of power, then yaw control can be combined with pitch control. If the aircraft is yawing, then the wing on the outside of this yaw movement will go faster than the inner wing. This creates higher lift on the faster wing, resulting in a rolling movement, which helps to make a turn.
Controlling airspeed speed has been shown to be very difficult with engine control only, often resulting in a fast landing. A faster than normal landing also results when the flaps can not be extended due to loss of hydraulics.
Another challenge for pilots who were forced to fly an aircraft without functioning control surfaces was to avoid the phugoid instability mode (a cycle in which the aircraft repeatedly climbs and then dives), which required careful use of the throttle.
Because this type of aircraft control is difficult for humans to achieve, researchers have attempted to integrate this control ability into the computers of fly-by-wire aircraft. Early attempts to add the ability to real aircraft were not very successful, the software having been based on experiments conducted in flight simulators where jet engines are usually modelled as "perfect" devices with exactly the same thrust on each engine, a linear relationship between throttle setting and thrust, and instantaneous response to input. More modern computer systems have been updated to account for these factors, and aircraft have been successfully flown with this software installed.[1] However, it remains a rarity on commercial aircraft.
Accidents and incidents involving commercial aircraft
Controls damaged by engine failure
- LOT Polish Airlines Flight 5055, an Ilyushin Il-62M, on 9 May 1987. According to the Polish investigatory commission, the cause of the crash was the disintegration of an engine shaft due to faulty bearings inside engine No. 2, which seized, causing extensive heat. This in turn caused the consequent damage to engine No. 1, rapid decompression of the fuselage, and a fire in the cargo hold, as well as the loss of elevator controls and progressive electrical failures. Zygmunt Pawlaczyk decided to return to Warsaw Okecie Airport using only trim tabs to control the flight of the aircraft. He lost his struggle to land about 5 km from the runway in the Kabacki Forest. All 172 passengers and 11 crew members perished.[2]
- United Airlines Flight 232, a McDonnell Douglas DC-10, on 19 July 1989. A fan disk in the No. 2 engine fractured, severing most of the flight controls. Dennis Fitch, a deadheading DC-10 instructor who had studied the case of JAL Flight 123, was able to help the pilots steer the aircraft using throttle differential. Despite the break-up of the aircraft on landing, 175 of 285 passengers and 10 of the 11 crew members survived.[3]
Controls damaged by structural failure
- American Airlines Flight 96, a McDonnell Douglas DC-10, on 12 June 1972. The failure of the rear cargo door caused an explosive decompression, which in turn caused the rear main cabin floor to collapse and severed flight controls. The pilots had only limited ailerons and elevators; the rudder was jammed. The number two engine also ran down to idle at the time of decompression. The aircraft landed safely at Detroit-Metropolitan Airport.[4]
- Japan Airlines Flight 123, a Boeing 747, on 12 August 1985. A faulty repair years earlier had weakened the aircraft's rear pressure bulkhead, which failed in flight. The vertical stabilizer and much of the aircraft's empennage was blown off during the decompression. The pilots were able to continue flying the aircraft with limited control, but after 32 minutes the aircraft crashed into a mountain, killing 520 of the 524 people aboard in the deadliest single aircraft disaster in history.[5]
- Air Transat Flight 961 on 6 March 2005, catastrophic structural failure: the rudder detached from the aircraft with a loud bang. The pilots regained enough control to land the aircraft safely.[6]
Controls damaged by explosive device/weapons
- Philippine Airlines Flight 434, a Boeing 747, on 11 December 1994. The hydraulics were damaged by a bomb in the passenger cabin.[7]
- DHL shootdown incident in Baghdad on 22 November 2003. The Airbus A300 DHL aircraft, hit by a surface-to-air missile, was the first jet airliner to land safely without any hydraulics using only engine controls.[8]
Maintenance/pilot error/Sabotage
- The aircraft designer Roy Chadwick was killed on 23 August 1947 during a crash on take-off of the prototype Avro Tudor 2, G-AGSU, from Woodford airfield. The accident was due to an error in an overnight servicing in which the aileron control cables were inadvertently crossed.
References
Notes
- ^ "Active Home Page". Past Research Projects. NASA. Retrieved 2006-06-01.
- ^ Gero 1997, p. 199.
- ^ Gero 1997, p. 210.
- ^ Gero 1997, p. 125.
- ^ Gero 1997, p. 189.
- ^ Flight 961 - Official accident report www.bst.gc.ca Retrieved: 1 June 2010
- ^ Flight 934 - Aviation Safety Network aviation-safety.net Retrieved: 1 June 2010.
- ^ Aviation Safety Network aviation-safety.net Retrieved: 1 June 2010.
Bibliography
- Gero, David. Aviation Disasters. Patrick Stephens Ltd (Haynes Publishing). Yeovil, Somerset. 1997 1 85260 526 X