Old 13th Mar 2019, 23:01
  #163 (permalink)  
Join Date: Jun 2001
Posts: 549
Originally Posted by gums View Post

@FCeng from my post

What I am implying is that the MCAS was put in the plane in order to satisfy certification requirements and just old basic aero you would expect from an airliner. i.e. it was not installed or required in previous 737 models.

I have a good plot of the longitudinal pitch authority of the plane I flew 30 years ago. At one point (AoA) the stab looses nose down control authority with a "neutral" stick command. Ditto for my VooDoo, and most of us could feel the stick getting lighter if we were pulling hard. No feedback of any kind in the Viper or Airbus to let you know how close you wereto losing it.

So what I am hearing about the new 737 is that as we get close to a certain high AoA that the plane looses some amount of pitch authority that is normally commanded by the pilot. Worse, it may even enter the "pitch up" mode where no pilot command will lower the nose.

Gums sends...
In respect of the MCAS/certification requirement.

Gums, the 101 had a particular issue related to the T tail configuration, which gave endless entertainment and some great movies. The Max's issue stems from the nacelles of the LEAP donks, and they are destabilising at higher AOA. The engine weight is beneficial to static stability, lift from the nacelle forward of the Cp (sans nacelle) is not.The nacelle lift is fairly non linear to AOA, a substantial portion is induced by the vortex flow off the nacelle body at higher (not high for a 16 or 18 etc...). I suspect that the MLA is also playing into the equation, but don't have info on the criteria for the triggering of that, however the MLA whenever active reduces the static stability of the aircraft.

The Max is not the first Boeing or 737 to have some quirks, the classic STS is understood, but the NG did as well but was cured with good revision. The B767 had its own variant of quirks with 25.173 compliance which was cured aerodynamically by add ons and then by refined design.

The unfortunate state of affairs is that the envelope area that is protected by 25.173 that resulted in MCAS which has the potential to cause more serious handling problems is as much a compliance/box ticking exercise as it has to do with acceptable safety of flight. Without direct knowledge on the PSCP that was undertaken, that being proprietary to TBC, one can assume from the information released that the issue arises during a wind up turn profile at aft CG, which doesn't happen as often as takeoffs and landings occur with a single AOA vane as a sensor in the current system architecture. Put the reg in perspective, a century after Sperry starting handing out instruments for IMC flight... a Pitts usually has a little trim requirement, not much and is nice to fly; it sits where you point it until disturbed, dynamic stability is not an issue, VFR is fine, IFR is not too bad. Another single engine jet I fly only has a trim change for the gear and flap change; from 100KCAS to 450KCAS there is no appreciable trim change, and that is a delight to fly, VFR or IFR, it is statically weakly positive, but has good dynamic stability. Static stability was important in the days of lousy instruments, and hand flying, as a cue to what the driver was doing.

In a perfect world the planes would have linear responses that are just so, meeting the regs. To do that needs assistance or FBW, and the latter comes with it's own set of benefits and attendant risks. The B777 and 787 FBW systems are nice to fly, but are more importantly simple to revert to Cessna 150 state, but then MAS had a wild ride with even that system once upon a time. The Airbus FBW is nice for a pilot to fly when it is working properly, when it is not, it can be pretty entertaining getting the right result. [A logical weirdness in the 777 and 787 is that the plane is hand flown with a speed (AOA) referenced through the trim switch, but with the autothrottle in SPD mode as well... and underneath it all the planes still have a phugoid that is not taken out by the control law. That is just curious and makes the A320/330/340 etc nice to fly in comparison, other than in a crosswind].

FC84, great input.

25.173 Static longitudinal stability.

Under the conditions specified in 25.175, the characteristics of the elevator control forces (including friction) must be as follows:
(a) A pull must be required to obtain and maintain speeds below the specified trim speed, and a push must be required to obtain and maintain speeds above the specified trim speed. This must be shown at any speed that can be obtained except speeds higher than the landing gear or wing flap operating limit speeds or VFC/MFC,whichever is appropriate, or lower than the minimum speed for steady unstalled flight.
(b) The airspeed must return to within 10 percent of the original trim speed for the climb, approach, and landing conditions specified in 25.175 (a), (c), and (d), and must return to within 7.5 percent of the original trim speed for the cruising condition specified in 25.175(b), when the control force is slowly released from any speed within the range specified in paragraph (a) of this section.
(c) The average gradient of the stable slope of the stick force versus speed curve may not be less than 1 pound for each 6 knots.
(d) Within the free return speed range specified in paragraph (b) of this section, it is permissible for the airplane, without control forces, to stabilize on speeds above or below the desired trim speeds if exceptional attention on the part of the pilot is not required to return to and maintain the desired trim speed and altitude.
[Amdt. 25-7, 30 FR 13117, Oct. 15, 1965]

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