Acknowledgements: Sporty’s AIRFACTS/Mac McClellan

 (Ed. Note: Using a topical example, the following is extracted from Mac McClellan’ article which is available in full at It is notrelevant to the 737 Max exclusively, or even an aircraft of any type, or even only to pitch trim, as the principle would apply to any machine with any auto-operating element as part of its operator’s control systems - TB)

 “Non-pilot acquaintances often ask about the Boeing 737 Max and its MCAS system ..... I tell them MCAS moves the pitch trim to add force to the controls under some conditions of airspeed, CG and other factors. I also explain that if MCAS runs amok, as it possibly did in the two fatal accidents involving the 737 Max, the situation is no different from a trim runaway, a failure we pilots have always been trained to handle. 

 Many are amazed that the MCAS system doesn’t have multiple backup modes to detect failures and disable the system. I say that’s what the pilots are for. The pilots are the backup to the automated system in airplanes .... (but) to recover from a system failure, first you have to recognise it.

 When the pitch trim system runs away, the pilot has multiple methods of disabling it. The first line of defence is a button under the pilot’s thumb. There are also switches, and in most airplanes, a circuit breaker than can be pulled to totally depower the trim system.

 .... Critical systems such as autopilots and pitch trim are certified .... test pilots must recognize failure by some positive event, such as seeing the trim wheel moving un-commanded, or sensing an unexpected force on the flight controls. Once that positive identification is made, the test pilot must wait exactly three seconds before taking the proper action to disable the system. The FAA calls those three seconds “recognition time.” During the final stages of a landing approach, the recognition time is reduced to 1.5 seconds in the belief the pilot is paying closer attention during that critical phase of flight.

 When non-pilots hear this information, their mouths drop open .... (but) In my career of flying with so many test pilots .... the three seconds sounded totally logical to me .... (it) is lots of time to get the thumb of your flying hand on a button on the control wheel.

 .... At least one pitch trim runaway is part of every initial or recurrent simulator training program, .... often during high workload periods such as when configuring for landing when the flaps and gear extension are changing the stick force ...., (sometimes) during those brief periods of cruise flight, when the autopilot is flying and you are reading a checklist or studying the procedure for the next approach. Identifying a trim runaway in straight and level cruise with everything working fine can be the most difficult.

 .... (but) sim training is not the real world of flying. In the sim we are all primed at every moment for something to fail, because it usually does .... and that’s the root of the myth of the perfect pilot. During training and checking, we all must meet a standard that is essentially perfect. In the case of a trim runway, we must demonstrate without fail that we do in fact recognise the failure during the three-second period and react properly to recover .... 

 .... (in the real flying world,) pilots (are) the backup, both as monitor and then as asystem” to recover from the failure .... failures do happen, but in nearly all situations the crew reacts correctly, and the flight continues to a safe landing .... the public trust pilots, but unlike many of us who are pilots, the public knows there is no perfect pilot”.


Tony Birth