Throttles Only Control.P65

INTERpilot – 2004 THROTTLES ONLY CONTROL (TOC) 10 Steps to a Survivable Landing Following Loss of Normal Flight Control Captains Terry Lutz, Air Line Pilots Association, International and Brian Greeves, Hong Kong Air Line Pilots' Association Captain Terry Lutz Captain Brian Greeves Few emergencies in commercial aviation are more terrifying to the flight crew than loss of normal flight control. The July 1989 United Airlines DC-10 accident in Sioux City, Iowa and the November 2003 missile attack on the DHL A-300 departing Baghdad are examples where the crew lost all hydraulically powered flight controls. In both cases, the flight crew regained flight path control using throttles only, and were able to bring the airplane to a less than precise touchdown at an airport. Modern aircraft are exceptionally well designed, numerous systems failures, including rapid particularly from the standpoint of reliability and decompression, the crew had to deal with large pitch redundancy in the flight control system. Using failure oscillations and loss of directional control with throttles analysis techniques that consider all known failure alone. They were able to extend the landing gear, but modes and their subsequent effects on the flight as the crew explored what flight control remained, control system, modern flight control systems are they were unable to avoid hitting a mountain and 520 designed for a failure probability of 10-9 or less. Loss lives were lost. of flight control accidents that have happened over the The Boeing Company conducted their own tests after last 30 years have occurred because of an unpredicted this accident, and were able to successfully slow a event such as fan disk failure, aft pressure bulkhead Boeing B-747-200 from cruise configuration and speed failure, or loss of a cargo door. In each of the above to an acceptable landing configuration and speed, cases, the event severed hydraulic lines or jammed using throttle only control and the electric flap switches. control cables, leading to the loss of normal flight The aircraft industry reacts quickly to correct the control. cause factors in these accidents, and it is unlikely that In March 1974, a Turkish Airlines DC-10 departed those exact events will be repeated. Perhaps the key Paris and suffered the loss of an aft cargo door. In the thing that we can learn from studying them is that the resulting decompression, the cabin floor buckled and flight crew had some degree of pitch control available either jammed or severed control cables to the tail by increasing or decreasing thrust, and that differential mounted hydraulic actuators. The airplane impacted thrust could be used to control bank angle and the ground at high speed with the loss of 346 lives. heading. In April 1975, a USAF C-5A carrying 314 orphaned The Sioux City accident and the Baghdad missile attack Vietnamese children lost a rear cargo door which actually have much more in common: caused structural damage and loss of hydraulic power Eleven years ago, the Captain of the DHL A-300 to the tail. The crew continued to fly the airplane with happened to see a televised report about the Sioux the available flight controls on the wings, and regained City accident. pitch control with the throttles. The airplane flew successfully for over 30 minutes, but crashed short of the runway with the loss of 138 lives. In April 1977, the elevator on one side of a Delta L-1011 jammed in the full-up position on an instrument departure from San Diego. The crew instinctively used bank angle to control the pitch up, which created large pitch oscillations. Just as the airplane was about to stall, the Captain had the amazing insight to retard the wing engines to idle and firewall the centre engine. The crew learned quickly to control pitch with the throttles, and moved passengers to adjust the centre of gravity before accomplishing a successful landing. In August 1985, a JAL Boeing B-747 experienced Those of you who recognise this picture will know that this DHL failure of the aft pressure bulkhead. All hydraulic Airbus A-300 aircraft made it back to land in Baghdad thanks to the systems and most of the vertical tail were lost. With tremendous skill of the crew using Throttles Only Control (TOC) 32 Throttles Only Control Under extreme stress, he was able to recall the throttle To bring the airplane to straight flight, differential techniques and crew coordination that would be thrust should be used slowly. Thrust increase lags necessary to fly the airplane. Although they had never thrust lever movement, and the roll moment lags thrust practised theses manoeuvres before, in a short time, increase. If thrust is increased quickly, fuel slosh can they mastered this new skill and were faced with also become a problem to controlling bank angle. returning to Baghdad before one of the engines failed Once the airplane nears wings level, matching the due to fuel starvation. With a strong will to survive, the thrust parameters will stop the roll rate, but some mental tools available to use the throttles for control, Dutch roll oscillations may remain. Rather than trying and exceptional CRM skills, the DHL crew was able to to reduce these oscillations with thrust, it is best to let successfully land their severely damaged airplane. This them damp out naturally. was an outstanding demonstration of airmanship and At the same time you are trying to control bank angle, the question remains, could any of us do it! the airplane will likely be oscillating up and down in the long period or “phugoid” mode of motion. The magnitude and time period of these oscillations will depend on two things: the trim position and Centre of Gravity (CG) at the time normal flight control was lost. A full cycle of nose up, nose down, and back to nose up may take 1 to 2 minutes. If the CG is forward, the oscillations will be bit shorter, and take longer to damp. If the CG is aft, the oscillations will be a bit longer, and damp a little sooner. As you watch the pitch oscillations, you can get some idea of the eventual trim speed by mentally averaging the minimum and maximum speeds observed. To damp the pitch oscillations, apply thrust in smooth, but well-timed pulses. The pulses should be applied to stabilise pitch in an attitude for level flight. If the nose Today, we can provide far better information to pilots is above the horizon and falling, increase thrust briefly than a televised report. Following the United DC-10 to increase airspeed. If the nose is below the horizon accident in Sioux City, the National Aeronautics and and rising, decrease thrust briefly to decrease airspeed. Continue these pulses until the oscillations stop, then Space Administration (NASA) began extensive research watch airspeed carefully to keep it within 1 or 2 knots into a concept they call ‘Throttles Only Control’. Test of trim speed. Pitch is now under control. Pilot C. Gordon Fullerton and Flight Test Engineer Frank W. Burcham evaluated several transport category One key point made by NASA pilots and those few aircraft, both in the simulator and in flight, as well as airline pilots who have actually had to deal with loss of high performance military airplanes such as the T-38 normal flight control is that the learning curve is quite and FA-18. rapid. There will be a terrifying few minutes trying to damp the oscillations, and then confidence is rapidly When normal flight control is lost, the pilot will want gained that you can, in fact, control the airplane. The to continue flying the airplane with the normal flight ultimate question will then be: Can you descend, controls, but no matter how hard you try, the airplane configure, and successfully land? won’t respond. The pilot must make the mental and Since the trim system is likely frozen as well, the physical transition to the use of thrust as the sole airspeed achieved after damping the pitch oscillations means of flight control. The control moments due to is the speed you will have to work with. You can climb thrust are weak compared to normal flight control, and by increasing thrust and damping oscillations so that it almost looks like you have no control at all. But when the nose is in a climb attitude. NASA conducted their studies over the last 8 years, they found that for every airplane they studied, there Decreasing thrust and stabilising in a nose low attitude was sufficient control available with thrust alone to will allow you to descend. To change trim airspeed will achieve gross control of the airplane. Heading and require changing configuration or moving CG with fuel or cargo shifts. Use good knowledge of all systems, flight path angle could be controlled to within 3 to 5 both electric and hydraulic, that have an affect on flight degrees initially, and it improved with practice. control. Make changes slowly and incrementally. The airplane will likely enter two dynamic modes of Examine all the messages on the EICAS or ECAM to flight that we seldom see because our flight control completely understand the status of the aircraft. systems are so well designed. Those modes are the Configure early and be extremely careful when phugoid mode (longitudinal oscillations), and the Dutch extending the landing gear. In two cases with loss of roll mode (roll/yaw oscillations). If normal flight normal flight control, the crew experienced a pitch control is completely lost, the first thing the flight crew upset when the gear was extended. But after it was should attempt to do is bring the airplane to straight extended, the airspeed became easier to stabilise.

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