Link to Interesting article: “Can Boeing Trust Pilots?
by Mac McClellan, 11 March 2019, AirFacts.
Because the new MAX version of the 737 has heavier engines and other changes, Boeing added a system that under certain conditions of airspeed, CG location and weight, automatically moves the pitch trim to modify stick force. The pilot who is hand flying feels this as though he is pulling on the yoke and would naturally reduce pull force to lower the nose and angle of attack (AOA).
In the non-aviation media, this system is being called everything from new, to radical, to untested. In reality, nearly all airplanes larger than a basic four-seat piston single use some sort of device to alter the forces a pilot feels while maneuvering the airplane.
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This concept of adding artificial feel using the pitch trim has been around for years. It has been used to add stick force at high speed cruise where Mach effects can alter stick force as well as at higher AOA where stall margins must be maintained.
What’s critical to the current, mostly uninformed discussion is that the 737 MAX system is not triply redundant. In other words, it can be expected to fail more frequently than one in a billion flights, which is the certification standard for flight critical systems and structures.
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What’s critical to the current, mostly uninformed discussion is that the 737 MAX system is not triply redundant. In other words, it can be expected to fail more frequently than one in a billion flights, which is the certification standard for flight critical systems and structures.
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Though the pitch system in the MAX is somewhat new, the pilot actions after a failure are exactly the same as would be for a runaway trim in any 737 built since the 1960s. As pilots we really don’t need to know why the trim is running away, but we must know, and practice, how to disable it.
The problem for Boeing, and maybe eventually all airplane designers, is that FBW avoids these issues. FBW removes the pilot as a critical part of the system and relies on multiple computers to handle failures.
Boeing is now faced with the difficult task of explaining to the media why pilots must know how to intervene after a system failure. And also to explain that airplanes have been built and certified this way for many decades. Pilots have been the last line of defense when things go wrong.
I heartily recommend reading the entire article. Particularly because today, we are all flying in Fly By Wire aircraft most of the time.
Other developments of interest in the USA, where Boeing is the manufacturer and the FAA is often the first to certify. It turns out the FAA can, and does, outsource critical parts of the testing and certification decision making to the very manufacturers they’re supposed to be evaluating critically, under a policy implemented ten years ago.
“FAA employees warned as early as seven years ago that Boeing Co. had too much sway over safety approvals of new aircraft, prompting an investigation by Department of Transportation auditors who confirmed the agency hadn’t done enough to ‘hold Boeing accountable’.”
Link to Bloomberg article “Boeing Had Too Much Sway in Vetting Own Jets, FAA Was Told”, By Peter Robison and Alan Levin, March 17, 2019, 7:31 PM PDT, Updated on March 18, 2019, 2:22 AM PDT
A USA Grand Jury and US Department of Justice are also investigating. Link to Wall Street Journal article.
Link to Wall Street Journal article “Prosecutors, Transportation Department Scrutinize Development of Boeing’s 737 MAX; A grand jury’s subpoena seeks broad documents related to the jetliner”, by By Andrew Tangel, Andy Pasztor and Robert Wall, Updated March 18, 2019 11:38 a.m. ET