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The Roller Coaster method of recovery from runaway stabiliser trim

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The Roller Coaster method of recovery from runaway stabiliser trim

Old 19th Nov 2019, 13:13
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The Roller Coaster method of recovery from runaway stabiliser trim

The following extract was published in FSINFO.ORG flight safety newsletter on 18 November 2019.

A 737 captain on a U.S. airline, who asked for anonymity to speak without permission from his employer, described his own extensive experience as a former test pilot of moving the tail manually.
He said that with the 737 tail at full nose-down position and at maximum design speed, it is "nigh impossible for a normal human to move the manual trim wheel in the nose up direction. The forces are too strong."

Dennis Tajer, an American Airlines captain and APA spokesman, recently replicated that flight situation in a simulator, deliberately inducing an MCAS-style nose-down pitch at high speed, though still within the normal flight range. He was able to move the wheel only "a couple of inches, but not enough."

Tajer said that if the MAX is pitched down toward the ground, it gathers speed all too easily.

"The 737 is a slippery airplane," said Tajer. "When you put the nose down, it wants to accelerate very quickly."


He and his co-pilot in the simulator were able to recover control by using an old piloting skill called the roller-coaster technique that's no longer in the manuals: letting go of the control column to ease the forces, then cranking the wheel, and repeatedly easing and cranking.

This so named "roller coaster" or "Yo Yo" method was first mentioned in a 1961 Boeing Aero magazine concerning recovery from a runway stabilizer trim in the Boeing 707. The same technique was then applied to the first of the Boeing 737's and was subsequently published in the 737 Pilot Training Manual (PTM) now called the Flight crew Training Manual.

Extract from the Boeing 737-200 Pilot Training Manual February 1982 page 04.80.31. Edited for brevity

Runaway and Manual Stabiliser - Recovery from Severe Out-of-Trim" In an extreme nose-up out-of-trim condition, requiring almost full forward control column, decelerate, extend the flaps and/or reduce thrust to a minimum practical setting consistent with flight conditions until elevator control is established. Do not decrease airspeed below the minimum maneuvering speed for the flap configuration. A bank of 30 degrees or more will relieve some force on the control column. This, combined with flap extension and reduced speed should permit easier manual trimming.

If other methods fail to relieve the elevator load and control column force, use the "roller coaster" technique. If nose-up trim is required, raise the nose well above the horizon with elevator control. Then slowly relax the control column pressure and manually trim nose-up. Allow the nose to drop below the horizon while trimming. Repeat this sequence until the airplane is trim.

If nose-down trim is required, slowing down and extending the flaps will account for a large degree of nose-up pitch. If this does not allow manual trimming then the reverse "roller coaster" can be performed to permit manual trimming." (I read somewhere it was called the Yo Yo manoeuvre)

Boeing "Airliner" magazine published in May 1961 discussed the above subject as it applied to the Boeing 707 by stating: "To trim the stabilizer manually while holding a high stick force on control column. As the airplane changes altitude, crank in the desired trim change. Correct airplane attitude after a few seconds with elevators. Relax stick force again and crank in more trim. Repeat this procedure as necessary until proper in-trim position of stabilizer is established."

These manoeuvres were used in the 1950-60s. Yet, for some inexplicable reason, Boeing manuals have since deleted what was then - and still is - vital handling information for flight crews.

In November 1975, Boeing published its 737 Instructor Pilot Guide. With 122 pages in booklet form it covered the 737 Basic and Advanced versions of the 737. Page 51 covered procedures for countering a runaway and manual stabilizer. This was conducted in the air if required and gave the following advice:

INITIAL CONDITIONS.
Altitude: 5000 ft above terrain (minimum)
Airspeed 210 to 280 kts
Configuration: clean
Stabilizer trim: note.

.Perform on the ground first. Do not combine with jammed stabilizer. Do not use electric trim on ground to engage brake.
Request change of airspeed. Do not continue any runaway stabilizer trim action beyond 2 units from in trim.
PROCEDURE.
Cut out stabilizer trim cutout switches. Oppose trim with smooth control movement. Retrim manually.
Out-of-Trim-Recovery Methods
Use manual trim. Accelerate or decelerate into trim. Use roller coaster (least desirable). Bank airplane for nose up out-of-trim.
Clean up
Stow stabilizer trim handles. Set stabilizer trim switches to normal
Discussion.
Cutout switches to cutout, even before brake engages. If stabilizer brake or cutout switches fail to stop runaway, grab trim wheel and hold. Smooth elevator: do not jerk.
……...…...……...…………………………………………………………………………………………………………… ….
All this was conducted airborne; maybe because simulator fidelity could not be assured in those days.
Instructor technique varies as we all know (sometimes to our cost.) The following suggestion may help when introducing this exercise in the simulator.

On the ground, run the electrical stabiliser trim to full down or full up. Then have the student manually wind the stabiliser trim to a setting of five units while counting the number of full turns of the stabiliser wheel it takes to reach mid range (approx. 5 units). This gives the student some idea of the time it takes to wind the stabiliser wheel. Explain winding the wheel is easy since there are no air loads on the stabiliser. Once that part of the exercise has been completed have the student practice locating and actuating the stabiliser cutoff switches in darkness to simulate night conditions. Remember Boeing allowed only three seconds during certification for the pilot to actuate the cutout switches once a runaway stabiliser is sensed. Therefore prompt locating and actuation of the stabiliser cutout switches is vital.

To demonstrate the roller coaster method in the simulator set airspeed to 250 knots. With one pilot flying have the other pilot deliberately run the stabiliser forward while the flying pilot attempts to maintain altitude. Eventually the flying pilot can no longer hold level flight (by then the stabiliser trim is probably around 2 Units nose forward. Select stabiliser cutout switches to cutout. Attempt to raise the nose well above the horizon using both pilots if necessary. At that point relax the pull force and as the nose down moment starts to take effect have the flying pilot rapidly wind the manual stabiliser trim backwards before the aircraft starts to dive. Boeing call this "aerodynamically relieving the airloads".

Repeat the manoeuvre until the stabiliser trim position allows effective elevator control. Depending on the severity of the initial nose down runaway trim, it may require several roller coasters before elevator control is effective and stabiliser trim is back in mid-range.

Author Captain D.P.Davies comprehensively covers the subject of large trim changes, failure cases and Mach number effect on stabilizers, at pages 38 to 42 in his fine book "Handling the Big Jets," A good case for current airline pilots to buy his book as it is still the best on the market, IMHO

Last edited by Judd; 19th Nov 2019 at 13:33.
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Old 19th Nov 2019, 14:22
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From an earlier thread:

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Old 19th Nov 2019, 16:02
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Widely discussed in Boeing advice on "aerodynamically relieving airloads" using manual stabilizer trim
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