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Old 12th Dec 2021, 07:38
  #67 (permalink)  
fdr
 
Join Date: Jun 2001
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Originally Posted by safetypee View Post

1. Assuming that all crews (any crew) will be able to fly an aircraft, in any situation, etc, does not match the reality of human behaviour; each of us understand situations differently, according to our view at the time, influenced by experiences, knowledge, training.


2. The 737 has changed; from a conventional swept-wing low-thrust design where controls (and trim) were harmonised with the thrust pitching moment, and a simple (by modern terms) dual channel AP and AT.

3. The need for Cat 3 auto-land, automatic trim up, an advanced dual FGS requires alternative procedures for GA, for AP/FD, single or dual modes; added complexity. Increased demand on awareness, knowledge and recall, different skills.

4. More recently increased engine thrust; the manual control system has to cope with a larger pitching moment, but unable to match previous flying qualities exactly - but ‘the crew will manage’ (differences training). And a final mitigation, an afterthought at the end of the checklist ‘If TOGA thrust is not required, then …’; more demand on awareness, decision, action; more complexity in identifying a safety alleviation.

5. Within this, a critical assumption that the aircraft can be flown with the same basic techniques taught ab-initio; control for pitch, trim to reduce force, a sequential action.
Uplinker identifies the 737 weakness - pitch and trim simultaneously, where any delay adds workload, getting being behind the aircraft.
1. That would seem to be a reasonable expectation from the self-loading freight, those under the flight path, the company and the regulator...

2. the -100 and -200 series were quite nice performers, In particular ROC. the thrust line was slightly less offset than the CFMI blenders, but doing the maths on the moment arm gives little difference in the resultant, except the thrust level is increased... The -300, -400 and -500 are vanilla designs. The -600, -700, -800 are same again, and the -900 adds some geometry issues that should be a point of concern, but, they all fly the same. Then we got the Max, and essentially the Max is the same except for the high alpha lifting body effect that arises from the cowling. Boeing went for reasons of their own logic that are incomprehensible to me with a repurpose of a system to resolve that particular test point. Aerodynamic modification was an alternative and would have been a benefit in weight and drag as well, but, the repurpose was the solution that was incorporated, and the rest is history.

Neglecting the high alpha Max issue, every B737 behaves the same in principle, in fact, exactly the same as a B747, B744, B757, 767... [The 777 and 787 in normal modes have the PFCs giving a trim reference speed which the plane will settle to, however, they also are both FBW systems that have a phugoid, and that means the pilot may well get to see more sky than expected in a lightweight, double-tap of the TO/GA levers]. The A300-600, and the A310 are spectacular in their ability to get out of sorts in trimming. Perpignan's A320 and many others have shown that the mindset change from normal to direct law in Airbus aircraft can end up being challenging for the driver, while the remainder to use manual THS comes up on the ECAM, history suggests that this little gem in a situation of cognitive overload kind of gets missed, the old "I couldn't hear your go-around call over the sound of the gear up warning horn". Lears, Hawkers, Falcons, Gee whizzes etc (other than the FBW variants (falcon goes C* ways, like the Bus, Boeing likes C*U... and gets a phugoid for its pains) all of these need trimming in Go Arounds, in fact, they usually all need trimming all the time. About the only plane that doesn't need trimming is a Soko Galeb, from stall to 430KIAS you can leave the trim alone and fly by fingertip. At the other end of the scale, a T-28 with the -86R at 1425hp doesn't do anything without trimming in all 3 axis, at the same time, so having 5 hands makes for smooth formation aerobatics... The bottom line, changing the noise level in almost all aircraft needs a trim input to manage the outcome, and aircraft with large speed differences between low and high-speed envelopes, and with large CG ranges will always have a stabilizer that has a high authority compared to the elevator. [neglecting F4's... F8's, A7's, etc... ] The ability to get out of trim is not a unique factor of the B737.

3. Cat 3. itself didn't require the nose up trim input, which is a dual autopilot issue, where it has to achieve a fail passive outcome, If a 3rd AP had been added, 3rd power source, independent instrument transfers etc... then fail-operational would have been possible which does not involve a nose-up trim bias input. B747s, triple or dual autoland could be conducted, Triple avoided the out of trim if a manual landing was made with a late AP disconnect. The B737 dual AP has to be fail passive, hence the trim bias.

4. for the variants, the moment arm change for the tail offsets the thrust change on most heavier models. the shorter body big blender types will usually want some additional thumb flickering for a full thrust GA, but all need trim input.

5. Attempting a GA in an A300 or 310 by a sequential application of elevator then trim is going to be.a great airshow. bring popcorn. Same with a light B763 with big GEs or PWs. Doing the same with a T28B or D will give a neat tight barrel roll, [ well, things get fun]. Hit the TOGA 2 times on a light B744 and try to fly with elevators only and you will get sore arms, and still see lots of sky. MD11's were fun too. A large thrust change in most jets requires prompt elevator AND stabilizer trim inputs, commensurate with the rate of change of the moment and the magnitude of the moment change. nothing magical in that.

Swept wing high-speed transports have a high ratio of Vmo/Vs, fairly large Cp changes from transonic Mach effects, and wide CG ranges, and almost invariably have a limited elevator authority relative to stabilizer authority. (Boeing always used to desire a plane that could be flown by the driver using just the control column in an engine failure, which while a wonderful sentiment is itself a lousy real-world proposition.... just as if the thrust couple changes it necessitates a trim change (trim=stab...) a thrust asymmetry requires an appropriate trim change for the yaw, that is a rudder input. [countering roll only with aileron in almost all modern and not so modern jet transports results in the potential for an increase in stall speed where yaw has not been controlled].). going right off track, Cp rearwards shift with increasing mach No is a relatively gentle affair, however, the term tuck certainly comes to have a meaning when a shock develops on the lower surface of a wing, which results in a rapid change in Cl, and a reduction in the flight path angle from the loss of lift.


The B737 is just a plane, it is not my favorite Boeing, it has sloppy ailerons, and the dual-acting single servo rudder control valve was about as bad as the. concept of MDD's fail-safe stab screw jack design, or airbus's A300 vertical stab secondary structure failsafe design... . but push and pull is conventional, even with the MAX where the crew is let into the secret of the MCAS system being fitted.

Between C* and C*U, the C* flies nicer when it works. The C*U as Boeing implemented it has the need for the driver to use the trim switches to re-index "U", and so the reversion from normal to degraded laws is not a difficulty. The Airbus obviates the trim wheel use by the driver in normal flight, but then when stuff gets rowdy the crew has to have the presence of mind to use the stab trim (THS) lest they conduct an impromptu Aresti series.
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