Before Alaska Airlines Flight 261 crashed, the pilot reported problems with the stabilizer. How does the stabilizer control the plane? Have other accidents been caused by stabilizer malfunctions?
[Alaska Airlines, Flight 261, an MD-83, crashed into the Pacific Ocean near the Channel Islands, off of Port Hueneme, California on January 31, 2000, with the loss of all 88 souls on board.]
There has been much confusion in the press media about the design and function of the “Horizontal Stabilizers” that are found on all modern jet airliners. One example, of that confusion, by Lisa Stark of ABC news:
That is not correct. Pitch trim (the nose up/down tendency of the airplane), is achieved by moving the entire horizontal stabilizer up or down.
When a jetliner takes off, its pitch trim must be set by the pilot so that it is within the fore and aft safety limits, considering the gross weight and CG (center of gravity) of that particular airplane. The CG is determined by how much weight is in the plane, and where that weight is located. The pilot sets that pitch trim with split-switches on the control yoke, after he has been given the correct setting by the load planning dept. of his airline. He moves those switches up, to achieve a more nose down trim, or he moves them down, to achieve a more nose up trim. When those switches are activated by the pilot, the entire horizontal stabilizer, on the tail section of the airplane, moves up or down.
If the load planning dept. has made its calculation correctly, then it takes only minimal effort for the pilot to lift the plane off the ground, once the takeoff safety speed has been reached, by pulling back on the yoke. As the plane continues to accelerate during the climb, the pilot usually makes pitch trim adjustments by pressing up or down on those split-switches on the yoke. He does that because the amount of force, he must hold on the yoke, varies as the speed changes. The pitch tendency of the plane is “in trim” when the pilot can take his hands off the yoke, without any noticeable change in up or down movement.
There are movable surfaces, attached to the trailing edge of the horizontal stabilizer, but they are not trim tabs; they are elevators. When the pilot pulls back on the yoke, he causes the elevators to move upward. That creates a higher air pressure on top of those elevators, as it simultaneously lowers the air pressure on the bottom side of those elevators. That, in turn, pushes the tail down. When that happens, the angle of attack of the wings is increased. The greater that angle of attack (the angle of the wing in relation to the angle of the air flow that the wing is passing through), the more lift that is generated by the wings. When the amount of that lift exceeds the weight of the plane (lift is greater than gravity), the plane begins to climb.
Both the autopilot, when it is engaged, and the human pilot make adjustments to the up/down position of the horizontal stabilizer throughout the flight, to keep the pitch in trim at all times. That serves three purposes:
· To relieve the strain on the human pilot. If the stabilizer is out of trim, for a given speed and configuration, then the pilot has to use constant muscle power to override the tendency of the plane to pitch up or down more than desired.
· To keep the elevators in line with the angle of the stabilizer. Doing so minimizes aerodynamic drag, which reduces fuel consumption.
· To ensure that the plane will not suddenly pitch up or down, when the autopilot is disconnected. Such sudden pitch oscillations, depending on the degree of severity, can be dangerous to unbelted passengers and crew.
Malfunction of the horizontal stabilizer trim system constitutes a serious flight control problem. How serious, depends on the kind and degree of malfunction, the weight and CG of the plane at the time of the malfunction, and the design of that particular airplane. The failure of the stabilizer trim system is much less dangerous in a 747-400, for example, because the elevators are so large that the pilot can safely land the plane, even if the stabilizer has jammed at its extreme limit (either up or down). With the assist of hydraulic power, the physical force required by the pilot, to overcome the extreme out-of-trim condition, is well within the capability of the average person.
Some past accidents caused by out-of-trim stabilizers:
1962, June 3:
An Air France B-707-328 began its take-off roll at Orly Airport (the old Paris airport – new one is De Gaulle). When the pilot pulled back on the yoke, the nose raised up only for a few seconds. When it dropped down, the pilot tried to abort at a speed of nearly 200 mph. The plane ran off the runway, hitting approach lights and then a house. It finally stopped 1,500 ft. beyond the runway end. Fire quickly engulfed the fuselage, killing 130. Two flight attendants, sitting in the rear, were the only survivors.
The horizontal stabilizer was found trimmed 2 units towards the nosedown position. That made it very hard for the pilot to lift the nose off the ground at the appropriate take-off speed, causing him to think he had lost pitch control, which meant the plane would not fly.
Accident investigators concluded the servo trim motor, for the horizontal stabilizer, had failed and that cause the wrong take-off trim setting. The failure of that motor would also have prevented the pilot from trimming it back to the take-off position, once he felt that strong resistance to his pulling back on the control yoke.
They never could determine why the servo motor had failed. The terrible irony was that the pilot could have forced the plane to fly, with a lot of muscle power – to offset the negative forces generated by erroneous nose down trim. But, because he didn’t immediately have that kind of data information, and thus didn’t know it could fly, he made the only decision he could: to abort.
Over the years, pilots have referred to this as the first “runaway stabilizer” accident.
1963, February 12:
A Northwest B-720-B, while departing from the Miami Airport, entered a steep dive after passing thru an altitude of 17,500 ft. The descent speed was so great, the plane exceeded its structural limitations and disintegrated in flight. All 43 onboard perished.
The stabilizer trim jackscrew was found positioned to within 3/32 inch of the aircraft nosedown mechanical stop. That is the stopping point of the jackscrew when it is operated electrically. No evidence of arcing, burning or electrical overload on any of the generators was ever found. The pilot, Capt. Roy W. Almquist, age 47, had a total pilot time of almost 18,000 hours, but only had 150 hours in the 720, his first pure-jet airliner.
Multiple factors contributed to this accident (which is the case in most accidents), but the most important factor was that the pilot could not retrim the stabilizer back to a noseup position, because the excessive forces stalled the jackscrews.
The flight was being vectored around extensive thunderstorm activity as it departed and was subjected to moderate to severe turbulence. That made it difficult for the captain to read his attitude gyro, and also caused the plane to pitch up alarmingly, when they hit a severe updraft. Apparently, to get the nose down, because he believed the plane would soon stall, he trimmed the stabilizer to the full nosedown position as he also pushed forward on the yoke. When they exited the updraft, the plane rapidly pitched over to begin a terrifying descent. That sudden pitch-over produced negative G-forces that probably took the captain’s hands off the yoke, which then allowed the elevators to remain in the full nose down position, for several seconds. That, combined with the full nosedown trim of the stabilizer, caused an extreme descent rate from which it soon became impossible to recover.
The CAB (which investigated accidents until the NTSB was formed in 1967), recommended:
· Increasing the horizontal stab trim motor torque capacity to preclude motor stalling.
· Modify elevator control force characteristics to eliminate stick force lightening (the pilot would tend to overcontrol if the feedback “feel” is too light).
· Create a “Turbulence Mode” for the autopilot that would deactivate the Mach trim and stabilizer trim systems.
· Expedite the installation of improved attitude indicators (adding a blue color code to the area above the horizon) to help the pilot in maintaining attitude control even at high pitch and roll angles.
1963, July 12:
Capt. Lynden E. Duesher was in command of United flight 746, a B-720, from San Francisco to Chicago. Near O’Neill, Nebraska, he observed turbulence on his radar. He decided to try and “climb above it,” so he began the ascent to 41,000 ft. from his current 35,000 ft. As they passed thru 37,500 ft., they encountered severe turbulence and then downdrafts and then an updraft, causing the plane to approach a stall.
[For the weight and power of that airplane, he was approaching what pilots call “coffin alley,” meaning that as the plane climbs, the maximum (mach buffet) speed becomes lower while the minimum (stall) speed becomes higher. The range between those two limiting speeds narrows, until they eventually converge. That becomes the absolute altitude limit of a given plane. Any turbulence produces additional G-force (same as the plane suddenly becoming a lot heavier) and the plane will stall and fall if the speed exceeds either side of that coffin alley envelope.]
The nose pitched up to 15 degrees, and then up to 40 degrees, even though both were pushing full forward on the yoke. The plane stalled and rolled to the left and dove for mother earth. Now the nose was pitched down to 35 degrees. Duesher tried to use the stab trim to help pull the nose up, but it wouldn’t work. [I don’t know what position the stab trim was in at that time, but apparently not full nosedown, as in the other accidents.] He had pulled the power back to idle and extended the speed brakes. That, plus the fact that he wasn’t pointed straight down, kept the plane from exceeding the speed of sound, though it came awfully close. As they approached 15,000 ft. the heavier air enabled them to gingerly apply back pressure on the yoke, while simultaneously increasing engine thrust. They managed to level off at 14,000 ft.
At Chicago, the flight recorder was pulled to examine the actual stresses on the plane. The tracings appeared to be very similar to those found on the FDR of the Northwest 720 crash near Miami (above). Yet, a full examination of the plane revealed no structural damage; not a rivet was popped. The difference, was pilot technique.
After that near disaster, the stall and mach buffet margins were widened on all jet aircraft to preclude a plane getting into that situation again, where severe turbulence narrows the “coffin alley” margins so instantly that the pilots do not have time to avoid a high altitude stall.
1963, November 29:
A Trans-Canada Airlines DC-8, 54F, took off from Dorval Airport, near Montreal. Five minutes later it crashed, leaving a crater in the ground, killing all 118 on board. Impact speed was over 500 mph. They found the horizontal stabilizer trim setting at 1.65 to 2 degrees nosedown and concluded it had move to that position via hydraulic power (the normal force that moves that stabilizer on the DC-8). They also guessed that the pilot had intentionally trimmed it to that nosedown position -- which was abnormal for a plane taking off.
From that, they speculated that the PTC (pitch trim compensator) had malfunctioned and extended too far as the plane’s speed accelerated during climb. That would cause the control column to move aft, and the plane to pitch up abnormally. The pilot probably then moved the stab trim to that nose down setting to compensate, causing the plane to suddenly pitch over into a dive. If he immediately tried to pull back on the yoke, to stop the dive that the nosedown trim produced, the hydraulic-driven jack screws that move the stabilizer, would have stalled. The only way to then trim back to a nose up position was to release pressure on the yoke, which would have allowed the nose to pitch down even more – a luxury they could ill afford since the upset started at only 6,000 ft.
The plane had no flight data recorder, so they could never be certain if the PTC had malfunctioned, setting up the pilot for the fatal over-control situation. Two weaker theories were also suggested as to why the pilot would intentionally trim to that nose down setting:
· That icing of the pitot system had caused the airspeed indicator to read low.
· That the gyro horizon had become inaccurate, showing a higher nose up attitude than they really had.
The lack of a flight data recorder made it impossible to completely discard those two weaker speculations.
1964, February 24:
An Eastern Airlines DC-8 crashed into Lake Pontchartrain about 5 minutes after taking off from the New Orleans Moisant Airport. All 58 on board perished. The water was only 20 ft. deep, yet only 60 % of the wreckage was recovered, because the breakup was so extensive. The FDR tape was too damaged to help the analysis. Instead, they used the maintenance records of that plane, and of other DC-8s, to conclude that the pilots had trimmed the stabilizer to the full nosedown position to counter the excessive noseup attitude that, in turn, was caused by a malfunctioning PTC that had extended too far. Then, when the upset occurred, they could not trim the stabilizer back to the noseup position because the severe forces, generated by their pulling back on the yoke, stalled those jack screws.
They tried putting the engines into reverse (inboard engine reversing in-flight was allowed on the DC-8), and that nearly worked; the plane was almost level when it hit the lake.
Accident investigators found other instances of misrigging of the PTC, during the course of that investigation. A bushing was installed upside down on the Eastern plane, which would have caused the PTC to extend even further.
As a result of this accident, modifications were made to the DC-8 stabilizer trim system:
· A warning light was installed to alert the pilots when a PTC malfunctions and begins extending too far.
· The nose down travel limits of the horizontal stabilizer was reduced.
· The PTC actuator bell-crank arm was redesigned.
· Changes in flight crew and maintenance training in how to deal with PTC unwanted extension or complete malfunctions.
After these accidents, changes were made in pilot training. It became a “No No” to use horizontal stabilizer trim to ride out turbulence of any kind. The new procedure was to disconnect the autopilot and seek to maintain a “reasonable” – not “perfect” -- attitude, using only the elevators. Maintaining speed and/or altitude were to become secondary considerations; maintaining attitude was primary in moderate to severe turbulence, to prevent jet upset.
1978, February 10:
A Beech 99, flight 23, for Columbia Pacific Airlines, climbed steeply after takeoff from the Richland Airport in Washington, to an altitude of 400 ft., then stalled and crashed. All 17 on board perished.
Another Beech 99 crashed after its horizontal stab trim “ran away” to the full nose down position, while the plane was in cruise flight. It nosed over into a dive from which it did not recover. And, a 727 pilot, years after the change in turbulence procedures that flowed from the 1960s accidents, used stab trim, up and down, to try and compensate for rough air he encountered at 28,000 ft. He too, did a high dive and managed to pull out only 400 ft. above ground. Then, he shot back up rapidly (because his stab trim was too far noseup) and then suffered a second upset (again, because of misuse of stab trim). The second time he recovered at a higher altitude. I guess you could say he was learning how dangerous excessive stab trim could be. ALPA kept him from being fired outright by negotiating a deal for him to move back to the flight engineer’s seat for the remainder of his career.
There have been other reasons for upsets and loss of control:
1967, June 23:
A Mohawk Airlines BAC 1-11, crashed near Blossburg, Penn., with the loss of all 34 onboard. The plane took off from Elmira, NY, at 1439 EDT, and was cleared direct to Harrisburg, Penn. Acknowledgment of that clearance was the last communication received from that aircraft.
ATC gave another clearance, at 1444 EDT, to climb to 16,000 ft. The pilots, by that time, were attempting to cope with the loss of pitch control. The CVR tape shows they tried to respond to that clearance, but it was never received by NY center.
The FDR revealed the plane reached approx 6,000 ft., descended slightly and then climbed up to approx 7,500 ft., leveled off briefly and then dove down to approx 4,000 ft., then back to above 5,000 ft, then it dove again, until it crashed.
NTSB Selected comments on the CVR:
1445:15 CAM -1 "We lost all control! -- we don't have anything!"
1446:37 CAM-1 "What have we done to that damn tail surface, ya have any idea?"
CAM-2 "I don't know, ah, I, I just can't figure it out."
1446:44 CAM-2 "Ah, we've lost both systems."
1446:47 CAM-1 "I can't keep this--(#)-- from (unintelligible), all right, I'm gonna use both hands now."
1446:54 CAM-1 "Pull 'er back, pull 'er (untelligible) [sic] power!"
1446:55 CAM-1 "Both hands, back, both hands!"
1447:10 CAM-1 "PULL BACK!"
1447:11 CAM-1 "I've gone out of control!"
1447:17 . . . END OF RECORDING
For more information on inflight fires, read the FAQ Swissair
1973, July 22:
A Pan Am 707-321 B, plunged into the ocean about 30 seconds after takeoff from Papeete, Tahiti. It started its descent after initiating a left turn. The wreckage was in water over 2,300 ft. deep and the French investigators could not recover the flight recorders nor much of the wreckage. Thus, they could only guess at the cause. Horizontal stab trim is one possibility, though I think spatial disorientation is a more likely explanation. (It was nighttime over water with no lights.)
1985, February 19:
A B-747 SP, flown by a China Airlines Capt., suffered an engine failure while cruising at 41,000 ft. The Capt. left it on autopilot too long. The autopilot tried to maintain that altitude, which was ultimately impossible at that weight, with only 3 engines functioning. As it approached the stall, because the speed kept decelerating, the Capt. finally disconnected the auto pilot. He was not prepared, because he had failed to trim in rudder to compensate for the asymmetrical thrust condition; the autopilot was maintaining wings level by the use of aileron and spoilers only.
[Autopilots normally do not control the rudder in climb, cruise, or descent. They use only the ailerons, spoilers, elevators and horizontal stab trim.]
When he hit that disconnect switch, the plane rolled rapidly and entered a dive. Although the plane exceeded the speed of sound, tearing parts off and causing major structural damage, the Capt. was able to make a recovery at a few thousand feet over the Pacific Ocean, after he broke out of the clouds and could see his attitude via outside visual reference. There were, incredibly, only two serious injuries to the 274 passengers and crew.
The tail fell off a turboprop airliner because the elevator control rods were made out of aluminum instead of steel. When one of the rods failed, the elevator was loose and began to flutter, causing excessive loads on the horizontal stabilizer, which then broke off.
I also mention the takeoff crash of the UAL DC-8 freighter, due to full nose up stab trim, in my Editorial on CRM/CLR
[All emphasis is that of the author]
Robert J. Boser