Boeing 737 Manual Stabiliser Trim - an Historical Fact
The latest issue of Aviation Week & Commercial Aviation (Dec 23 2019 - January 12, 2020) includes an article Boeing 737 Pilots Focus on Modified Procedures.
One paragraph states: “The new 737 training modules emphasize that pilots may need to use two hands to crank the wheel during a runaway trim scenario. It also says “unloading” the stabilizer – attempting to reduce airspeed and take the counterintuitive step of not pulling back on the yoke even though the aircraft is trimmed nose down – may be necessary to move the trim wheel.”
I first flew the Boeing 737-200 in 1977 and the Pilot Training Manual for that aircraft explained quite clearly how to use the manual trim in event of a runaway electrically operated stabilizer trim. However, following the introduction of the Boeing 737-300 and subsequent models, Boeing, for some unknown reason, withdrew those directions and instead inserted an amended version which included the words “in extreme cases it may be necessary to aerodynamically relieve the air loads to allow manual trimming.” The amended directions did not say precisely how this was to be accomplished and it was left to pilots to interpret what Boeing meant. Pilots with English as second language would have little idea what Boeing meant.
The following extracts are from the Boeing 737-200 Pilot Training Manual dated 1982 page 04-80.31 and was included in all earlier 737-200 PTM. Under the heading of Abnormal/Emergency Procedures it stated:“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 pressure and manually trim nose up. Allow the nose to drop below the horizon while trimming.”
Contrary to the advice mentioned by Sean Broderick of “take the counterintuitive step of not pulling back on the yoke even though the aircraft is trimmed nose down,” you can see the Boeing advice is to initially raise the nose well above the horizon with elevator control.
Boeing Airliner magazine published in May 1961, discussed the roller coaster technique in detail as it also applied to the Boeing 707. It was also known as the YoYo technique.
If the crews of the two fatal MAX accidents had access to the above advice in their manuals it is possible they may have been able to control the runway trim. But their manuals did not include using the “roller-coaster” technique to manually regain trim control, because Boeing had elected to remove that advice from all Boeing 737 models.
It is not surprising that the FAA recently appears to have failed to notice that Boeing had changed the original wording of the 737-200 Pilot Training Manual. Most of their inspectors had not been born in 1961 when the roller coaster technique was first introduced on the Boeing 707.
Finally, here is an extract from the November 1975 edition of the Boeing 737 Instructor Pilot Handbook. Page 51 and titled Runaway and Manual Stabilizer.
Out-of-trim recovery method.
Use manual trim
Accelerate or decelerate
Use roller coaster (least desirable)
Bank aircraft for nose up out of trim.
These methods were used when practicing out of trim recovery while airborne.
The FAA are merely re-inventing the wheel if they think their latest advice on using manual stabiliser trim to recover from a runway trim is new. We knew all about the problem sixty years ago and so did Boeing.