Throughout a normal flight, a pilot controls an aircraft through the use of flight controls including maintaining straight and level flight, as well as turns, climbing, and descending. Some controls, such as a "yoke" or "stick" move and adjust the control surfaces which affects the aircraft's attitude in the three axes of pitch, roll, and yaw. Other controls include those for adjusting wing characteristics (flaps, slats, spoilers) and those that control the power or thrust of the propulsion systems. The loss of primary control systems in any phase of flight is an emergency. Aircraft are not designed to be flown under such circumstances; however, some pilots faced with such an emergency have had limited success flying and landing aircraft with disabled controls.
Control system failures resulting in disabled controls have resulted in a number of aviation incidents and accidents. Some incidents occurred where controls were not functioning correctly prior to take-off, others where the failure developed during flight. A loss of control can occur when an unrelated failure, such as an engine failure, causes damage to control related systems. For instances, in several incidents an engine broke apart, causing the failure of main and redundant hydraulic systems, which disabled all control surfaces. Some or all controls can become inoperative from extreme weather conditions, due to collisions, due to poor maintenance or mistakes made by maintenance workers, as a result of pilot error, due to failures of the flight control system, or due to design or manufacturing flaws.
In normal flight, maneuvering an aircraft requires some combination of controls, which are often interactive in their effect.
A basic means of controlling an aircraft with disabled flight controls is making use of the position of the engines. If the engines are mounted under the centre of gravity, as in underwing 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 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 cannot be extended due to loss of hydraulics.
Another challenge for pilots who are forced to fly an aircraft without functioning control surfaces is to avoid the phugoid instability mode (a cycle in which the aircraft repeatedly climbs and then dives), which requires 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.
Incidents where disabled, damaged, and/or failed control systems were a significant or primary cause of the accident.
In these incidents, a failure of propulsion systems (engine, fan, propeller, pumps) caused damage to control systems. (Engine mounting failures are covered under structural failures, below.)
In these incidents, a failure of structural components (bulkheads, doors, struts, mounts, spars, hull) subsequently damaged control systems.
In these incidents, there was a failure of control system components themselves (e.g. cables, hydraulics, flaps, slats, ailerons, rudder, stabilizer, trim tabs, auto-pilot). (Control system fatigue failures are here, but improperly installed or incorrectly adjusted controls in the next section.)
In these incidents, the failure of control system components was caused by improper installation or adjustment of control systems components by maintenance personnel.
In these incidents, pilot error resulted in control system damage.
These incidents describe mid-air collisions that mainly damaged control systems of at least one of the aircraft, which may or may not have been recoverable.
In the Charlie Brown and Franz Stigler incident on 20 December 1943, a Boeing B-17F Flying Fortress of the 527th Bombardment Squadron was tasked with carrying out a bomb run on Bremen, Germany, in formation with other B-17Fs. Before the bomber released its bomb load, accurate flak shattered the Plexiglas nose, knocked out the #2 engine and further damaged the #4 engine, which was already in questionable condition and had to be throttled back to prevent overspeeding. This caused the plane to fall back from the formation and left it vulnerable to enemy attack. The B-17F was then attacked by over a dozen enemy fighters (a combination of Messerschmitt Bf 109s and Focke-Wulf Fw 190s) of JG 11 for more than ten minutes, causing the pilot to lose consciousness and putting the B-17F into a steep dive. The pilot later regained consciousness and recovered the plane from the dive. Further damage was sustained from the attack, including to the #3 engine, reducing it to only half power (meaning the aircraft had effectively, at best, 40% of its total rated power available). The bomber's internal oxygen, hydraulic, and electrical systems were also damaged, and the bomber had lost half of its rudder and port elevator, as well as its nose cone. The crew on board were also wounded with one of them being killed. After being escorted by a Luftwaffe Messerschmitt Bf 109 G-6 to be out of German airspace, the B-17F landed at RAF Seething. [22] [23] [24] [25]
On October 10, 1928, U.S. Army photographer Albert William Stevens and Captain St. Clair Streett, the chief of the U.S. Army Air Corps Materiel Division's Flying Branch, flew the XCO-5 experimental biplane to achieve an unofficial altitude record for aircraft carrying more than one person: 37,854 feet (11,538 m); less than 1,000 feet (300 m) short of the official single-person altitude record. [30] Stevens snapped photographs of the ground below, warmed by electrically heated mittens and many layers of clothing. At that height the men measured a temperature of −78 °F (−61 °C), cold enough to freeze the aircraft controls. [31] When Stevens was finished with his camera, Streett found that the aircraft's controls were rendered immobile in the cold, with Streett unable to reduce throttle for descent. The aircraft's engine continued to run at the high power level necessary for maintaining high altitude. Streett contemplated diving at full power, but the XCO-5 was not built for such strong maneuvers—its wings could have sheared off. Instead, Streett waited until fuel was exhausted and the engine sputtered to a stop, after which he piloted the fragile aircraft down in a gentle glide and made a deadstick landing. [31] An article about the feat appeared in Popular Science in May 1929, entitled "Stranded—Seven Miles Up!" [31]
NASA personnel at Dryden Flight Research Center worked on the design of an aircraft control system using only thrust from its engines. The system was first tested on a McDonnell Douglas F-15 Eagle in 1993, piloted by Gordon Fullerton. [33] The system was then applied to a McDonnell Douglas MD-11 airliner, and Fullerton made its first propulsion-controlled landing in August 1995. [33] Later flights were made with the center engine at idle speed so the system could be tested using the two wing-mounted engines, simulating the more common airliner layout. [34]