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Boeing 737 MAX Accident Analysis

By Major Rob Robinette

I was a US Air Force Flight Safety Officer and conducted several accident investigations. I also have over 7000 hours as a pilot in the 737. The primary reason Boeing's new 737 MAX crashed twice was poor training which led to a severe lack of basic pilot knowledge of the 737. The "Official Cause" was that Boeing added a new trim assist system called MCAS and did not document this change in any pilot manuals. When an external sensor failed the MCAS received bad info and caused large, unwanted changes in the stabilizer trim. We call this Runaway Stabilizer trim. Boeing's MCAS was blamed for the accidents to deflect blame from the unbelievably incompetent pilots. It does not matter if MCAS caused the runaway stabilizer trim because the runaway can be solved the same way as any other runaway trim.

The procedure for stopping runaway trim is a memory item in the Quick Reaction Handbook (QRH). The QRH is a small book full of emergencies and what to do about them. All 737 pilots from the 737-200 to the 737 MAX are required to memorize several steps on how to handle runaway stabilizer trim so they can solve the emergency without referring to the QRH book. Pilots are required to state these "memory items" during their semiannual check ride (flight test).

The 737 stabilizer trim system, including MCAS, rotates the entire stabilizer (circled in blue above). Diagram by The Air Current.

Not only did the four accident pilots not know the memory items for runaway trim, they didn't even know that runaway trim was in the QRH! If any of the four pilots on those two doomed aircraft simply knew to look in the QRH for the Runaway Stabilizer trim procedure they would have saved their aircraft. As a matter of fact, on the flight just before one of the fatal flights the aircraft had MCAS runaway trim and again, the two pilots in the seats did not execute the memory items for Runaway Stabilizer or call for the procedure in the QRH. Luckily, a third pilot in the cockpit jump seat knew to flip the Stabilizer Trim Cutout Switches and saved the aircraft. It is absolutely shocking that six 737 MAX pilots could be so poorly prepared for this simple emergency.

Runaway Stabilizer trim emergency procedure from the QRH (Quick Reaction Handbook). The items in the box are "memory items" that all 737 pilots must memorize so they can handle the emergency without having to open the QRH book. Note the third and fourth steps, "STABILIZER TRIM CUTOUT SWITCHES...CUTOUT" (turn off the electric trim) and "STABILIZER TRIM WHEEL...GRASP AND HOLD". "GRASP & HOLD" means stop the trim wheel with your hand. "TRIM MANUALLY" means turn the trim wheel by hand.

Another fact that is overlooked in discussions about the MAX crashes is that the electric trim switches on the pilot yokes (steering wheels) override all other trim changes. If the pilots would have simply kept using their thumb to activate trim they could have overridden the MCAS system and maintained control of the aircraft. I know that statement is mind blowing but it is absolutely true.

Dual trim switches on the pilot yoke. Both switches must move in the same direction to activate the trim. This switch overrides all other trim commands, including MCAS.

The sad thing is there is an even simpler way to stop runaway stabilizer trim, the manual trim system. Any of the four pilots could have simply stopped the manual trim wheel from turning with their hand and saved the aircraft. The manual trim wheels are 13 inch diameter wheels in the cockpit near both pilots knees that spin noisily when you change the trim. They make a surprisingly loud, "clunka, clunka, clunka," sound when they turn. Boeing made the trim wheels noisy on purpose, so both pilots would know that trim was being applied and alert them to runaway trim. The wheel has a wide, smooth outer rim made so you can stop the wheel from turning by applying pressure with the palm of the hand. This wheel was spinning loudly during the MCAS induced trim runaways (it can be heard on the Cockpit Voice Recorder) and none of the pilots knew they could simply stop the trim with their hand.

Boeing 737 Manual Trim Wheel with handle folded out. You can easily stop runaway trim by stopping the trim wheel with your hand. With the wheel stopped you can change trim manually by folding out the handle (shown out in pic above) and turning it by hand. The white stripe makes sure the pilots know the wheel is turning.

The Captain of one of the aircraft asked the First Officer to, "Try manual trim!" The First Officer immediately replied, "It's not working!" which means the First Officer did not know the Captain wanted him to use the trim wheel--again, lack of very basic knowledge. The First Officer didn't know that "manual trim" means stopping the trim wheel with his hand and flipping out the folding handle and turning the wheel by hand. I do not know why the Captain didn't follow up and tell the First Officer to stop the trim wheel with his hand.

When the pilots were fighting the trim the QRH memory procedure says to turn the Stabilizer Trim Cutout Switches to "Cutoff." These switches are on the center console between the pilots, accessible easily by either pilot. Flipping those switches would have turned off the electric trim and ended the emergency. All 737 pilots must know by memory how to stop runaway trim, you just flip a switch or stop the wheel with your hand.

These two fatal accidents were caused by the lack of very basic 737 knowledge of the four fatal pilots. All four of them should know the name of every emergency procedure in the QRH so they know what procedure to call for when an emergency occurs. All four of them should have memorized the first four steps of the "Runaway Stabilizer" emergency procedure. It is totally mind blowing that none of the four pilots knew they could save the aircraft by putting their hand on the trim wheel to stop the runaway trim. Including the flight before the fatal flight that had MCAS runaway trim we have six 737 MAX pilots that didn't know something that every 737 pilot must absolutely know--that Runaway Stabilizer trim is in the QRH and you can stop it with the Stabilizer Trim Cutout Switches or by putting your hand on the trim wheel.

Boeing was blamed for the two MAX accidents because it's politically incorrect to blame the foreign pilots, but that is what killed two aircraft full of people--the pilots. This isn't easy for me because no pilot wants to blame another pilot for a fatal accident but people need to know the 737 MAX is a safe aircraft when flown by normal, knowledgeable pilots. I know this runs counter to official findings but it is the real truth. Runaway stabilizer trim is a very simple malfunction to fix. Ask any 737 pilot in private about runaway trim and the 737 MAX crashes and they will tell you the same thing--they were easily avoidable if the pilots would have executed the Runaway Stabilizer trim emergency procedure. Modern airliners are fairly easy to fly but when something goes wrong, knowledge, skill and leadership are required to bring the aircraft home safely--that's why airline pilots make the big money.

Major Rob Robinette
USAF Retired



Comments and corrections are always welcome at robinette at comcast dot net.


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