B737NG:FLIGHT CONTROLS, HYDRAULIC, FLIGHT INSTRUMENTS Q's

[Cap Loko]

Dear fellow aviators,

I have a few questions regarding B737-NG. Would like to know the answer to enhance my understanding of these systems.

-1.What is the significance of the difference in setup of system A and B standpipes in the hydraulic reservoir?; Only in case of a leak of engine-driven pump A will fluid level be maintained (until 20%) but if a leak occurs in any other pump (sys A EMDP and sys B EMDP and EDP), pressure is lost.Why do have both systems a different setup?

-2.Why is mach trim applied to elevators but not stabilizer instead?
-3.Is there any posibility of a Mach trim runaway and if yes what course of action (if any) should be taken as there is no QRH drill for this ?
-4.I.c.o. Mach trim fail, how come the airspeed limit is .82 and not .615 ?

-5.Flap maneuver speeds: The flap symbols on the Airspeed Indicator represent the Recommended Maneuvering Speeds for flap selected.

A captain I fly with told me that the flap symbols represent MINIMUM maneuvering speeds and thus is selecting to fly at speeds in the middle of two symbols; for example '1'at 180K '5' at 160K, he will fly at 170K with flaps 5 selected.

My understanding from the books is that for example when the '1' symbol is at 180 K, you should fly 180K with flaps 1. (Our company procedure -which I will not question- is to fly at the nearest highest 10 knot digit).

What is correct?


Many thanks in advance,
Loko

edited for question numbering

[blackmail]

hello cap loko,

interesting questions indeed.

i have no clue if my answers are correct, but here some reflections about the matter:
1) hydraulics. boeing did away with standpipes in sys.B, b737ng, because, from past experience, maybe they think it is not necessary any more. with massive sys.B leak you will still have enough fluid for ptu operation. in general, if something is deemed not necessary & or not required, boeing & any other aircraft manufacturer as a matter of fact, will not install it on cost/certification basis alone.

2) mach trim. computer calculations & flighttesting probably demonstrated that flying with mach trim inop was safe till 280kts/M.82. that the system moves the elevator & not the stab trim is maybe just the reason as to prevent a stab trim runaway.

3) flying at maneuverspeed is according to boeing, safe with adequate margins above stickshaker even with 25°bank + 15° overshoot.
i recommend reading boeing training manual, chapter general info,"flap usage"

[Justin Cyder-Belvoir]

1. Sys B has a standpipe which holds approx 24% of the system fluid for PTU operation of LE slats, flaps and autoslat. If the leak is in Sys A you will lose all the fluid as there is no standpipe there.

2.The Mach trim system defelects the elevator; this creates a force on the elevator. That force is in turn creates a torque which is sensed, I beleive,by the feel computer which generates a sugnal to the stab trim motor to move the stab to zero the torque and hence the force. Thus the stab "follows up" the elevator.

3. The bugged speed is the Vman for the flap setting i.e at 210 kts Flap 1 is selected, fly the 1 bug. When Flpa 5 is selected, fly the 5 bug etc.

[blackmail]

hello justin,

for the mach trim, you are correct that the elevator is moved slightly up & so controlwheel will move aft a little bit as well. as you say, this creates a force for a nose up trim input as in every other flying machine. now in the boeing & other aeroplanes with powered flight controls, this "force" is generated by "Q"feel(pitot pressure from pitot tubes on vertical fin),fed to "Q" feel computer,which uses hydraulic pressure to compress a calibrated spring. to get rid of this "spring" pressure, either the pilot or the autopilot, when engaged, electrically moves the stabilizer , which in turn, by mechanical interconnections to the spring mechanism, also called "the neutral shift" system, extends the spring back to neutral.
in case of mach trim operation however, this spring force is eliminated by moving the neutral shift mechanism altogether without any stab movement.
to summarize: pilot or autopilot controlwheel inputs need stab trim action
mach trim input moves controlwheel(aft) & elevator(up), but then moves neutral shift mechanism & not the stabilizer

[Cap Loko]

Thanks a lot for your effort and answers. I found all of them very usefull.

Regards
Loko