Sunday, July 7, 2019

EASA Tells Boeing To Fix 5 Major 737 MAX Issues

Source: Moon of Alabama

July 6, 2019


Boeing hopes to have the 737 MAX back in the air by December. But a list of five major requirements issued by the European aviation regulator EASA lets one doubt that the time frame can be kept.
EASA's checklist includes a number of issues that have been disclosed: the potential difficulty pilots have in turning the jet's manual trim wheel, the unreliability of the Max's angle of attack sensors, inadequate training procedures, and a software issue flagged just last week by the FAA pertaining to a lagging microprocessor. But the agency also listed a previously unreported concern: the autopilot failing to disengage in certain emergencies.
We will discuss the five issues below.

It is not clear if EASA will insist on all the points to be fixed:
"Any of these could significantly affect the return to service, but we don't know if they are actually going to become requirements or are they just items for discussion," said John Cox, a former 737 pilot who is president of the aviation consulting company Safety Operating Systems.
As usual the regulators will not tell Boeing how to fix the problems. Whatever solution Boeing offers for those items simply has to comply with the general demands the regulations make.

Some of the listed items seem to require hardware changes that will have to be applied to all 737 MAX and maybe even to the older 737 NGs.

Manual trim

We discussed the trim wheel issue back in May:
The 737 MAX incident also revealed a problem with older generations of the 737 type of plane that is only now coming into light. Simulator experiments (video) showed that the recovery procedure Boeing provided for the case of a severe mistrim of the plane is not sufficient to bring the plane back under control. The root cause of that inconvenient fact does not lie with the 737 MAX but with its predecessor, the Boeing 737 Next Generation or NG.
...
• The smaller manual trim wheels on the 737 NG make it more difficult to trim a runaway stabilizer back into a regular position.
• The larger stabilizer surface makes it more difficult to counter a runaway stabilizer by using the elevator which was kept at the same size.
• 737 NG pilots no longer learn the rollercoaster maneuver that is now the only way to recover from a severe mistrim.
EASA listing the trim wheel issue is the first official recognition of this problem.

The manual trim via the trim wheels is a necessary backup for the electrical trim system which relies on only one motor. If the manual trim can not be used in certain parts of the allowed flight envelope, Boeing has a severe issue at hand.

A 2015 EASA safety finding, previously discussed here, accepted the 737 MAX only because Boeing said that the manual trim wheel was operational even at higher speeds and when the electric trim cuts out. It also promised that its training material would cover the issue.

It is now known that the manual trim, especially at higher speeds, may require more force than an average pilot can apply. The general issue and the difficulty is still not mentioned in the current Boeing training material.

The trim wheel problem seems to be an item where the U.S. regulator FAA and the European EASA disagree:
The FAA has also previously denied that the trim wheel -- which is used to lift or lower a plane's nose during an emergency -- would cause delays.
It is hard to see how a manual trim system that, as Boeing told EASA, should be used where the electric system comes at its limits, can be acceptable without change, when it can not be moved in especially those cases where it should be used.

Angle of Attack Sensors

The second item on the EASA list is, as Bloomberg describes it, "the unreliability of the Max's angle of attack sensors".

Angle of attack sensor

The two angle of attack sensors on the MAX are not inherently unreliably. They are external sensors that are prone to get damaged. The original Maneuvering Characteristics Augmentation System (MCAS) that led to the crash of two planes, relied on only one of the two sensors. When that sensor got damaged, likely by a bird strike, MCAS trimmed the plane nose down into the ground. Boeing will now use both sensors to control the MCAS system. This may however not be enough.

During start and landing planes can collide with a passing flock of birds. In such cases both AoA sensors could easily get damaged. Many will remember that US Airways Flight 1549 landed in the Hudson river because bird strikes disabled both of its engines.

More modern planes than the Boeing 737 have 3 or 4 angle of attack sensors. Some other Boeing plane types have systems similar to MCAS. In addition to the AoA sensors they use an inertial system, acceleration and absolute position sensor, to determine if their MCAS like system should react.

It is quite possible that the regulators will now require a third sensor to be used to control the MCAS on the 737 MAX. If that is the case Boeing will likely prefer to add an additional internal sensor box to the plane instead of a third external sensor.

Please go to Moon of Alabama to read the entire article.
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