Back to home pageRunaway Stabilizer Procedure

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Contents

Illustrated technical information covering Vol 2 Over 800 multi-choice systems questions Study notes and technical information Close up photos of internal and external components A compilation of links to major 737 news stories with a downloadable archive Illustrated history and description of all variants of 737 Detailed tech specs of every series of 737 Databases and reports of all the major 737 accidents & incidents General flightdeck views of each generation of 737's Description & news reports of Advanced Blended Winglets Press reports of orders and deliveries A collection of my favourite photographs that I have taken of or from the 737 Details about 737 production methods A compilation of links to other sites with useful 737 content History and Development of the Boeing 737 - MAX A quick concise overview of the pages on this site

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Introduction

FAA officials have told airline safety officials and pilots unions in April 2019 that they are reviewing the Runaway Stabilizer procedure in the wake of the 737 MAX MCAS accidents. I have had a look back at my own collection of 737 QRH’s gathering dust in my library to see how this 52 year old procedure has evolved over the years.

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*** Updated 18 Apr 2019 ***

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The three marks on the fuselage indicate the upper, neutral and lower limit of travel of the horizontal stabilizer.

 

The evolution of the QRH Procedure

1967-1987

The whole procedure is just 3 lines long!
One memory item: “Stab Trim Cutout Switches____CUTOUT”
One Non-memory item: “Stabilizer __________ Trim manually”
And one note: Move control column smoothly as required to maintain desired airplane attitude.

Above left: QRH from 1975; Above right Stab trim cutout switches in a 737-200. They kept the same function until the MAX.

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1987-2000

After 20 years Boeing added a second memory item: “If runaway trim continues: Stabilizer Trim Wheel ______ Grasp and hold” and some helpful notes.

Above: QRH from 1987

Above: Classic stab trim cut-out switches


2000-2013

By the year 2000, the procedure had expanded to 4 memory items:

Above: QRH from 2000

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2013-Current

In 2013 the procedure changed to add in the step of disengaging the Autothrottle, and ending the procedure if disengaging the A/P stopped the runaway trim, taking it up to 5 memory items:

There are then 2 more non-memory items and a page of deferred items making a total of two full QRH pages, quite a difference from the 1967 3-liner!

Above: NG stab trim cut-out switches

Dennis Tajer of the Allied Pilots Association, which represents 15,000 pilots of American Airlines, has said of the FAA review: "I want them to be invasive. I want them to be aggressive. I want them to be obstinate about knowing all of the information, I want them to be more than trust but verify." He went on to say that even if the basic steps do not change, the spartan checklist language could be augmented with instructions such as those contained in Boeing supplemental materials: "It may take two pilots to manually trim the aircraft." "It may require elevator load alleviation in order to manually trim the aircraft." I have to say that I agree that more information in the QRH would be helpful.

US government officials told The Wall Street Journal in late May that the FAA is evaluating the emergency procedures not only for the Max, but also for older generations of the 737 including the NG. A Boeing spokesman responded in a statement "While we are working with the FAA to review all procedures, the safety of the 737 NG is not in question, with its 20-plus years of service and 200 million flight hours."

MAX Differences

Note that on the MAX, the MAIN ELEC and AUTO PILOT Stab trim switches now either switch will cut-off both main electric and autopilot stab trim. They are renamed PRI and B/U (primary and backup). The switches are guarded in the NORMAL position (switch up) and CUTOFF is switch down. The QRH procedure is the same for all 737s.

Above: MAX stab trim cut-out switches

 

The "Roller Coaster" Manouvre

The current FCTM states:

Manual Stabilizer Trim

If manual stabilizer trim is necessary, ensure both stabilizer trim cutout switches are in CUTOUT prior to extending the manual trim wheel handles.

Excessive airloads on the stabilizer may require effort by both pilots to correct the mis-trim. In extreme cases it may be necessary to aerodynamically relieve the airloads to allow manual trimming. Accelerate or decelerate towards the in-trim speed while attempting to trim manually. [This is known as the "roller coaster" or "yo-yo" manoeuvre!]

Anticipate the trim changes required for the approach. Configure the airplane early in the approach. When reaching the landing configuration, maintain as constant a trim setting as possible. If a go-around is required, anticipate the trim changes as airspeed increases.

The FCT 737 (PTM) c1982 described the "roller coaster" manouvre in better detail as follows:

Recovery from a Severe Out of Trim

Accelerate or decelerate the airplane to an in-trim airspeed. If a recovery must be initiated from an extreme nose-down out-of-trim requiring a high pull force, an increase in airspeed may relieve enough of the elevator load and control displacement to permit manual trimming. Do not exceed speeed limitation. If a recovery must be initiated from an extreme nose-up out-of-trim requiring a high push force, a decrease in airspeed may relieve enough of the elevator load and control displacement to permit manual trimming. It should be noted that the relationship between airspeed change and trim change do not remain constant. As airspeed is increased, trim change requirements decrease.

In an extreme nose-up out-of-trim condition, requiring almost full forward column, decellerate, 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 manouvring 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.

This operation is analagous to reeling in a big fish where the line is kept taught by keeping the pole-tip bent. Then, to reel in, the pole is dipped quickly while cranking fast to keep a pull on the line.

Flight crews should not hesitate to apply whatever force is necessary on the trim handwheel because the system is designed for large handwheel loads.

Airliner Magazine, May 1961 gives more details. It is based on the 707 but the system and technique are the same.

 

Further MAX Problems

A further problem with the MAX was discovered by the FAA around 17 June 2019. The FAA crew were simulating a Runaway Stabiliser during tests in Boeing’s MAX engineering flight simulator, also known as e-cab.

The FAA determined that:

  1. Line pilots would need more time to correctly diagnose the failure and execute the appropriate checklist.
  2. A computer chip malfunction could lead to uncommanded stabilizer movement during the emergency procedure.

One problem is that a specific flight control computer chip is processing data too slowly. It is uncertain whether the chip needs to be upgraded or if a software update can increase speed enough. Boeing is hoping for the latter solution as a chip upgrade will take much longer to impliment.

Boeing said “We are working through the software update and the potential implications for the timeline for the safe return to service of the 737 MAX fleet and resuming MAX deliveries. Our current assessment is we will submit our final certification package to the FAA in the September timeframe.”

This aspect of the FCC software update is not believed to be directly linked to the MCAS issues.

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