8 thoughts on “The 737 Max 8 Crashes”

1. I read an article yesterday, that part of the problem is that Boeing is marketing this plane as a mere upgrade of the 737. In fact, this iteration is more than an upgrade and more akin to a new plane. Upgrades requires less training for pilots, crew and maintenance than new planes. But they could not have expected as many orders if this were marketed as a new plane.

2. Well, they’re fixing a problem that occurred on the Air France flight that crashed into the Atlantic because the pilots kept holding the stick back despite the cockpit alarms blaring “stall, stall, stall”. So, the fix was:

   “Assume the pilot is a moron, so ignore them and shove the nose down anyway.”

   But as implemented, the fix is “The pilot is probably pulling the stick back to try and keep from dying in a fiery vertical crash, but assume they’re a moron and shove the nose down anyway.”

   They’re trying to fix stupid by adding more stupid.

   1. I agree, but that’s Lion Air.

      I’m confused about this Ethiopian Air, because of the claims of fire. MCAS wouldn’t cause a fire. So either the fire claims are in error, or something else occurred.

3. The chart is misleading as planes with fewer flights will appear to have more fatalities.

   1. Yeah I thought the exact same thing.

   2. Well, it’s like a win loss record. 20 and 5 has three more losses than 0 for 2, but that’s not what betters go by.

      In any event, Trump has just grounded the MAX 8 and MAX 9, in consultation with the FAA and others. The FAA said that new data, including satellite data, has indicated there is something going on. That implies that the eye-witness accounts of smoke and parts falling off were probably just imagined.

      One thing that came up regarding the previous and anonymous complaints was the planes sometimes tried to steeply nose over on autopilot, and the MCAS system isn’t engaged when autopilot is on.

4. I have an engineering observation, if this thread is still active.

   The reason Boeing added MCAS is that high angles of attack, the far forward engine mounting moves the center of lift forwards (from the nacelles), but also from the thrust vector * sine(AOA) creating a lift vector that’s far forward, pushing the nose up.

   But the MCAS runs nose down trim for 10 seconds, checks the AOA, and then repeats the cycle, potentially all the way to the travel stop on the jack screw.

   That’s the wrong way to do it.

   Fly an older 737 NG or Classic and measure the pitch performance at high AOA, perhaps using yoke forces as the standard. Then fly the 737 MAX at those same thrust/AOA points and measure it’s pitch performance. Adjust the change in the angle of incidence of the tail (the pitch trim) to make the required yoke forces on the MAX the same as the NG. This could be done with a simple lookup table or a curve fit, but the point is that for any combination of thrust and AOA, the difference between the two planes will be constant, and thus the MCAS system should just adjust the trim to a pre-determined point.

   That’s not what they did. What they did is act like the pitch force difference between an NG and a MAX grows over time, even if the AOA and thrust aren’t changing. They adjust, take a sensor reading, and possibly keep right on adjusting. The MCAS system is actively trying to fly the aircraft continually, instead of compensating for a built-in control difference between models.

   They could probably have added a cam under the jack screw that converts AOA or AOA and throttle setting to a predetermined trim change.

5. As long as I’m here, I might as well discuss the original design mistake of switching to the LEAP-1B engine, with its wider fan diameter that required the new engine location that caused the loss of margin in pitch at high AOA.

   They went from the CFM56’s 61 inch fan on the 737 NG series to a 69.4 inch fan on the LEAP-1B, which mean the engine wouldn’t fit under the wing.

   The bypass ratio went from 5.1 to 5.5:1 to 9:1 and the fan area went from 2922 sq inches to 3782 sq inches, a 29% increase. But the larger area obviously created a pretty serious problem.

   So I’ll throw out a crazy option.

   Use the Rolls-Royce Pearl 15 that’s rolling out this year for the new Bombardier Global 5500 and 6500 business jets. It has a 48.5 inch fan (1847 sq inches), but a low 4.8:1 bypass ratio and only 15,125 lbf thrust, compared to the LEAP-1B’s 27,000 to 29,000 lbs.

   So use four of them, podded two per nacelle like a B-52. The Pearl 15 is an outgrowth of the BR700 family, and Rolls-Royce is already pitching the BR725 in pods for replacing the B-52 fleet’s engines.

   The Pearl would give a fan area of 3694 inches, which is 97% of the LEAP-1B’s fan, and you’ve knocked 21 inches off the fan height, and are back to the vertical size of the JT8D on the original 737’s.

   In virtually all respects (seating capacity and range) Boeing has made the 737 fulfill the same role as the original 707, and a four-engine 737 would just complete the transition.

   But there’s still a slight shortfall in SFC because of the lower bypass ratio, despite the Pearl 15’s overall 43:1 pressure ratio, and there’s obviously added expense and complexity in maintaining twice as many engines.

   So keep both fans and get rid of one of the cores by enlarging an existing Pearl 15 core and adding a duct fed from between its high and low pressure turbines, which goes over to another low pressure turbine on the shaft that runs the second fan. The double-shaft/single core should end up with around a 9:1 bypass ratio, the same as the LEAP, with the same fan area as the LEAP, but with 20 inches less height.

   The LEAP-1B costs about $13 million. The CFM56-7 it replaced costs $11 million. The BR700 series is about $7 million, and the Pearl shouldn’t be much different because it didn’t bump the price on the new Bombardier aircraft that use it.

Comments are closed.