Top Letters And Comments, September 6, 2019

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Boeing 737 MAX

This MCAS problem reminds me of the “unintended acceleration” of the Audi 5000 from the 1980s. From most of the press reports, it sounded like the car would go full speed on its own, with the brakes failing to work at all, the drivers rendered as helpless passengers as the cars went completely out of control.

The reality was much more nuanced. First off, most of the affected drivers were of shorter-than-average stature. Many were new to the Audi, having come from bigger American cars. Right away that showed the driver was part of the problem, as how could the car know who was driving and react accordingly? In ALL cases the brakes worked just fine, and there was nothing wrong with the fuel system. Yet every driver insisted they were pressing hard on the brake pedal to no avail. If modern car computers were present with their data history, it would’ve showed the driver pressing hard on the gas pedal, and never pressing the brake pedal.

The problem? These drivers were mistakenly pressing the gas pedal, thinking it was the brake. The pedals were in not quite the same position as cars they’ve driven before, hence the confusion. It was compounded by an idle speed that, under some conditions, was higher than usual. When the driver shifted from Park to Drive, the forward lurch would startle the driver, and they would stab the ‘brake’ pedal but hit the gas instead. As the car lunged forward, adrenaline and panic would flow in equal measure.

Note that this only happened to automatic transmission vehicles, not manuals. And that provided a clue to the solution. In a manual, both feet are positioned on the pedals to shift from a standing start – there was no “pedal misapplication” (NTSB terminology). So, In addition to adjusting the pedal positioning and idle-speed programming, Audi invented the process of stepping on the brake pedal before one can shift out of Park. This feature is now standard on all automatic-transmission-equipped vehicles.

So, was the Audi at fault? Or was it bad drivers? Lots of owners thought the cars were just fine; they hated the drop in resale value. Or was it a combination of the two?

PS – Audi renamed the 5000 to the A5. Perhaps Boeing will have to do the same.

Kirk W.

Why was the MCAS software installed in the first place? Boeing has put forth various answers: so the 737 MAX would handle like earlier 737 models; to relieve airlines of the expense of flight simulator training for MAX pilots; to prevent stall under “unusual” flight conditions; to address a stability problem that manifests only under those “unusual” flight conditions.

History might be a clue. The original B737 was unstable in pitch. That too took Boeing by surprise in the late 1960’s, and airlines had to restrict passenger loading to the front of the cabin. They did so with a yellow ribbon tied between seats to block the rear of the cabin. I remember it well, because I worked at Boeing at the time and often flew 737’s from Seattle to Los Angeles.

The technical term is “longitudinal instability”. An airplane is longitudinally unstable when the center of gravity is behind the “neutral point”, which is the point about which changes in angle of attack produce no changes in pitch moment. Think of it as the point where lift changes caused by changes of angle of attack are concentrated. Boeing got the pitch instability sorted out with the B737-200 and subsequent models.

Until the B737 MAX. There are two obvious ways pitch instability may have been revived. One is the enlarged and forward engine nacelles, which must have moved the neutral point forward. The other is the distribution of fuel below the cabin floor, which may have allowed the center of gravity to shift behind the neutral point with heavy fuel loads, as at takeoff. The fuel is distributed in tanks each holding in excess of 1 metric ton. The B737-500 had four such tanks, and the B737-8 MAX has nine. Whether the aircraft is stable or not could depend on which tanks are filled and in which order they empty.

Longitudinal instability, center of gravity, and neutral point were entirely familiar to aeronautical engineers in the last century, but now I am not so sure. Boeing paid an engineer to address the stability of the B737 MAX, but he confused two modes of longitudinal oscillations: the low-frequency “phugoid” mode, and the “short-period” mode which alone can cause pitch instability. Evidence from the flight recordings of the two crashed B737 MAX’s indicates that they underwent violent short-period pitch oscillations.

I cannot know for sure, but I suspect that Boeing pilots flew close to pitch-unstable load conditions during early flight tests of the B737 MAX. I further suspect (based on years of teaching aerodynamics in the last century) that Boeing engineers had no knowledge of short-period pitch instability and instead became concerned about stall. The stated purpose of MCAS was to prevent stall, not instability. Of course, an unchecked pitch instability can cause stall when the aircraft pitches nose up, or a high-speed dive when the aircraft pitches down. Either way the end result is a crash. Solutions also date from the last century: increase the stabilizer area, move the fuel forward, or incorporate a proper stability augmentation system that actuates the stabilizer both up and down.

Steven C.

Poll: The FAA Is Being Asked to Block Supersonic Aircraft for Environmental Reasons. Should it?

Everything is bad for the environment. I think the worst is regulations like prohibiting supersonic flight. Instead, let’s identify what’s actually really bad, and regulate that. Otherwise, you are likely going to end up killing innovation and ultimately hurting the environment. Make a ground decibel limit? Maybe. Forbid booms that really hurt the environment? Okay. Speed limit for planes? Why? That’s just stupid.

Eric W.

The FAA’s authority does not include environmental protection. That sounds more like something the EPA would try to do.

Anonymous

Supersonic aircraft burn enormous amounts of fuel (therefore out larger quantities of pollution per passenger), emit excessive noise pollution the rest of the aviation community will get blamed for, and for what? So a privileged few can save a few hours of their precious time. No.

Anonymous

I’m not sure that is the FAA’s role, but I don’t think supersonic aircraft should operate because of the environmental impact.

Anonymous

It’s a dirty job but somebody has to do it. I don’t believe SSAC should be blocked. However, by refusing to consider possible disruption and requiring compliance with acceptable standards, the FAA (and aviation authorities in other countries) risk losing total control of regulations affecting aircraft, Cities, states and environmental agencies will force through regulations to fill the void.

Anonymous

If there are environmental concerns, then EPA should make the call.

Anonymous

Only approve electric mobility, we have enough pollution.

Anonymous

No; we need to grasp the technology and research and if proven safe for those in the craft and those on the ground then it should proceed.

Anonymous

Depends – How are they integrated into airspace and what airport infrastructure will be required? Will P-36 and P-150 noise regulations require changes?

Anonymous

If the boom is the problem, regulate the noise. If it is the emissions, regulate them. But just prohibiting supersonic flight only stifles innovation.

Anonymous

No, the military has been operating supersonic aircraft for many years, as well as the Concorde.

Anonymous

Has to be some sense, Boom aviation company claims they can make the economics work with newer technology than the Concorde.

Anonymous

If the economics make sense, some other country will do it.

Anonymous

Don’t have enough data to answer.

Anonymous

Not without credible evidence that it, in fact, poses a significant threat to the environment.

Anonymous

Depends on approved routes; should not be blocked, but regulated.

Anonymous

No, each aircraft should be evaluated for noise based on reasonable regulation.

Anonymous

I believe new technologies have taken a lot of the questions and bad info out of the question.

Anonymous

No. The fleet will be too small to be significant.

Anonymous

Catering to the UltraWealthy will continue our mass extinction.

Anonymous

Regulate the effects of the boom, not the fuel consumption.

Anonymous

Absolutely not. Allow the rich to spend their money so we can have jobs.

Anonymous

No, but environmental studies should be done.

Anonymous

Since not even a prototype experimental aircraft has flown, there’s no data to say either way.

Anonymous

Depends on environmental effects.

Anonymous

It should depend on how CO2 emissions per passenger-seat-mile compare with those of subsonic aircraft over the next few decades.

Anonymous

No. They should be permitted with valuable regulation to allow the US to remain the leaders of aviation while protecting the safety of the public.

Anonymous

Let’s see all the data first!

Anonymous

No! More useless regulation. No problem if the planes are at FL200 or above.

Anonymous

Ok over water, not ok over cities.

Anonymous

SS is not environmentally sustainable from noise and greenhouse gas emissions point of view.

Anonymous

The FAA’s authority does not include environmental protection. That sounds more like something the EPA would try to do.

Anonymous

Yes, unless they are far more efficient that current aircraft and that’s very unlikely.

Anonymous

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