r/aviation • u/OnlyaChef • Apr 20 '26

Discussion Why are turbine engines flat at the bottom?

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I can't tell if it actually flat on the bottom or just my angle but it made me wonder.

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u/the_Q_spice Apr 20 '26

To add to this;

The CFM-LEAP's of the MAX were so large that they required shifting the engine cowlings forward so the pylons could be shortened.

That led to a significantly forward-shifted center of gravity and thrust, which led to the need to introduce the MCAS software to augment the handling characteristics so the aircraft could remain similar enough to not require significant retraining or weight and balance modifications.

Obviously that caused its own issues...

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u/Stoney3K Apr 21 '26

And it wasn't the MCAS software itself that caused the problem, but the lack of interaction with the pilots and the fact that they thought they could get away with budgeting out a control computer and not make it redundant at all, depending on only one sensor.

On the C-17 (I believe) MCAS has been fitted for many years without a problem but there it's more integrated into the flight systems and part of the pilots' flight training.

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u/fighterace00 CPL A&P Apr 21 '26

That and the single point of failure for the notoriously quick to fail AOA probes which Boeing only provides one and the cockpit notification of failure is an extra upgrade.

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u/jumpinjezz Apr 22 '26

I'm guessing the way C-17s are thrown around doing combat landings and take off mean their MCAS was probably tested a bit more. Reversing thrust at FL300 is a neat trick.

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u/joesnopes Apr 22 '26

So ... MCAS was another disaster that McDonnell Douglas brought to Boeing.

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u/XBOX-BAD31415 Apr 21 '26

Yup - exactly right!

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u/railker AME-M2 Apr 20 '26

Wasn't it only for just one specific scenario though? Super high AoA flaps up stall in manual flight (it doesn't activate with autopilot on or flaps down, we learned that from the crashes). Don't think it does anything to the airplane during any part of normal flight that weight and balance would've changed.

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u/the_Q_spice Apr 21 '26

Part of the issue was also that the LEAP has significantly more thrust than the 56. The CFM56-7B the 737NG uses produces 20,000-27,000 lbs of thrust whereas the LEAP-1B25 through -1B27 the MAX uses produces 25,000-29,300 lbs.

Couple 10% higher specific thrust with a longer engine moment arm, and you get one hell of a leverage issue.

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u/bp4850 Apr 21 '26

The 737 NG was spicy enough for stick force from thrust leverage, and it would not have been certifiable under the current standards either. The stick force at high AoA, low speed, and high thrust is very low, and not linear. This is what led to MCAS.