737 200 questions

[atrflyguy]

Ok guys and gals I have been in recurrent training the last few days and have found some differences in system information I was given between my current employer and previous one. This has led to some interesting debate on whom or what is correct. The A/C type is the 737 200adv.

Question # 1 what does the Mach trim actually do? Aside from moving the control column back slightly, that we all agree on. Does it provide an extra 70lbs of force available to the pilot for recovery from a mach induced dive? Does it actually move the elevators aft to counter Mach tuck?

Question # 2 The APU requires time for cooling during ground start attempts and although there might be a slight difference in the time off required the question is, is there a time off for cooling required during start attempts in flight? Does this time off get waved just because it may be an emergency and you don’t want to wait 4 min after the first try before trying to start it again?

Question # 3 The APU will automatically abort the start sequence if it doesn’t reach 35% operating speed in what time? I have 60 sec from my old employer and am now being told 90 sec.

Question # 4 The thrust reverser isolation valves have an air ground sensor within the logic for open or closed. Is there two of them, one on the main gear and one on the nose gear?

Thanks folks and if you can give me some links to the info so that I can print it off etc that would be great too.

[c100driver]

Sorry I cannot post you a reference as my 200ADV manuals are long gone.

Question 1
It is to counteract Mach Tuck in case you diconnect the AP at speed and altitude so the aircraft is in trim, that is why it is called Mach TRIM?

Question 2 and 3
Which APU model is fitted?

Question 4
Our B732 ADV had one only on the RH MLG

[kenparry]

For Q1-3, like the previous poster, my manuals have gone & I don't remember.

Q4: the Boeing standard fit was main gear only, but some operators fitted (or specified) a noseleg switch as well. My recollection is, we were told it was possible to contact the ground with the reverser buckets if you selected reverse with a high nose-up attitude. The nosegear switch would, of course, prevent that.

[atrflyguy]

Yes but do the elevators actually move when the nutral shift happens in the controles?

As to which APU I don't know for sure at this time but I believe it to be a Garrett 85-129 if that makes sense.

[IslamoradaFlyer]

I agree with the previous posts regarding the trim. On the APU, all 72's I've ever operated had the APU air inlet in the MLG wheel well and the system was weight switched to the left MLG. You could not operate the APU inflight because of the weight switch, reduced air intake and the potential for a right MLG wheel well fire if there was an exhaust breech and proximity of the hydraulics.

I'm not familiar with any mods, but sure would be interested in hearing from operators who do.

[kenparry]

Islamoradaflyer:

The question is about the 737-200. I think you are talking about the 727 APU?

[HAWK21M]

The NLG Air-Grd sense was a mod carried out on some B732s,to ensure NLG on ground before deploying T/Rs.
All other aircraft have the RH MLG air-Grd sense.

regds
MEL

[Rainboe]

atrflyguy, one of the reasons for APU starter limitations is the load on the battery, not just the electric starter unit. One of our horror displays for the 747 was an APU battery where the starter limits had been ignored. For the Classic 747, the heat generated by a start was quite intense. Ignoring the limitations caused a complete battery metdown- the casing melted as well. I would think it is not different for the 737, so it is not just starter unit heat that is the problem.

[atrflyguy]

Yes I can see how that would make sense, but if it were being started using engine generator power on the busses and not just the battery would it make any difference? That the battery gets about the same amount of cooling in flight (I assume as the equipment cooling fan works in both) as on the ground would make this understandable for the time limitations between start tries. I was also under the impression that the limits were due to heat built up in the APU itself during the failed start attempt. We got to talking about the air flow past the intake and that in flight the flow into the intake for the APU might actually be less due to it tending to flow right on past the opening if you will. I would think then the limits would be greater in the air than on the ground except that my previous employer had it as no time required between start attempts while in flight.

Thanks all for the answers thus far and keep them coming as I am and could still learn something. Until I can find something from Boeing them selves as to weather the limits apply in flight or not I will keep looking for the exact answer to the question.

[Cough]

The APU always starts from the battery.

[captjns]

During APU start, the Battery Charger Relay opens to prevent the ground service bus from charging the battery during APU Start. The Battery Charger Relay protects the battery from overheat/melt down during heavy draw on the battery. As the battery is recharged after APU start, a thermal sensor in the battery tray will change the charge from a constant to a trickle to prevent battery overheat as well. The DC meter is monitored for a large draw to ensure the Battery Charger Relay opened when the aircraft AC busses are being powered.