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Guest rici

ALT FLAPS

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Hiya,just a quick question, I was reading the 737 aom at www.smartcockpit.com and the only section not on there is the hydraulics section! Firstly I noticed that if you turn the hydraulics off, then the flaps retract fully by themselves. Is that right?Secondly, I noticed that the ALT FLAPS switch only works with one of the main pumps on. I thought that when you switched the ALT FLAPS switch to arm, the sby hyd pump was activated, then the LE devices could be moved with the alt flaps position indicator, even with all four main pumps off.I was very supprised to see this system modeled, but I just wondered if it is working fully?How is the sby hyd pump powered?If the ALT FLAPS system works only with the hydraulics, how do you extend the flaps when all the engines fail, say due to fuel starvation? Sorry for the questions but I am a bit of a technical nut.ThanksRichard

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"Firstly I noticed that if you turn the hydraulics off, then the flaps retract fully by themselves. Is that right?"No... This is not correct, Richard. A definite fault here. I've mentioned it myself a few times already."Secondly, I noticed that the ALT FLAPS switch only works with one of the main pumps on. I thought that when you switched the ALT FLAPS switch to arm, the sby hyd pump was activated, then the LE devices could be moved with the alt flaps position indicator, even with all four main pumps off."Switching off all pumps should not stop the LE flaps extending with Stby Pump power. However, the extend process is a lot slower than normal. If I recall correctly, to help the extension process along, you should also select the flap lever to the intended position."I was very supprised to see this system modeled, but I just wondered if it is working fully?"I still think there are a few things to be ironed out in this area ;-)"How is the sby hyd pump powered?"115Vac power from the Right Transfer (Electrical) Bus. If your Right engine is shut down, this transfer bus should still be powered (by the Left Engine Generator/Left Transfer Bus, via the Bus Ties). DC power is also required for the motor control relays (I'll have to double check this, but I think you need to keep your Main Battery switch on)."If the ALT FLAPS system works only with the hydraulics, how do you extend the flaps when all the engines fail, say due to fuel starvation? "If your airplane is still flying/gliding at a respectable speed (say 150kts), your engines should still be windmilling fast enough to provide engine-driven hydraulics. Windmilling was not modelled in PMDG last time I checked (so you may not get any hydraulics). In this situation, there will also be no electrics to power the Elec Hydraulic pumps.Hope this helps.Cheers.Ian.

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Actually, the trailing edge flaps ALTN mode uses only electrical power. Ian, when you select the ALTN FLAP switch to ARM, the hyd valve to the TE flaps is closed. Setting the flap lever really doesn't do anything then.Only the LE Slats/Flaps are extended with standby hyd pressure, bu cannot be raised after extension.I would like to contend the fact that with a windmilling engine, you have sufficient engine driven hyd pressure. In all simulator sessions, it was apparent that engine driven hyd pressure was lost. The hyd system is quite frail on the 737, for example, simply selecting gear down in certain circumstances without engine driven hyd press on one side, causes such a pressure spike that the autopilot trips off -- just what you need in that late stage of the flight while single engine!!

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Another thing about the alternate flap switch, is that it has to be held to the down position. Its spring loaded off when toggled down(up is not spring loaded) ,momentarilly holding it down for a second and releasing it will only deploy the LE.Keeping it held down will also deploy the TE with the electrical motor. Its a good idea maintenence wise to keep the flap handle where the flaps have been run by alternate, as this helps relieve strain on the stby elec motor which is fairly weak, and helps in preventing inadvertent flap movement when hyd's is restored to normal.If you loose B hyd on takeoff for instance, the PTU (from sys A) can be used to retract the LE devices with the alternate flap sys(as long as the alt sys was not armed and the switch selected down: this will in effect close the PTU control valve.The only time the flaps should move by themselves is flap load releif 40-30 at 163kts, 30-25 at 176kts, and also decel 25-30 171kts,30-40 at 158kts as long as the flap handle remained at 40 .And autoslat operation when the flaps are in the 1,2 or 5 positions(SMYD commanded).All in all I think PMDG has done an amazing job with their 737.

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"Actually, the trailing edge flaps ALTN mode uses only electrical power. Ian, when you select the ALTN FLAP switch to ARM, the hyd valve to the TE flaps is closed. Setting the flap lever really doesn't do anything then."Remind me not to answer forum questions after coming off nightshift, Iz :-) Ignore (almost) everything I said.I think I was getting my flaps and possibly my aircraft mixed up, but the 737NG Boeing Maintenance Manual does actually tell you to move the flap lever to decreas the load on the TE electric motor. Ref CH 27-51-00. This definitely warrants further investigation ;-)"In all simulator sessions, it was apparent that engine driven hyd pressure was lost"A worst case scenario? Did the instructor say this always happens in real life? In these scenarios, I presume you didn't pull the associated fire handle??? :-)Thanks.Cheers.Ian.

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Ok thanks for your replies. Sorry, another question. If the standby hydraulic pump is powered throught the main electrical busses, then you will loose this pump in the event of a complete engine failure, I noticed that you would also loose the TE flaps electrical extension system too. So surly in the event of a double engine failure, the standby system would go too because the motor is not presurized. So you could not extend the flaps OR work the rudder. I know that the ailerons (apart from trim) and the elevator (in including trim) has manual reversion, but what about the rudder? Thats just standby hydraulics is it not?Thanks once again,Richard

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You still have the APU , it will supply all the power you need ,if it fails your stuck with the battery or batterys which will give you limited DC and sigle phase AC power(through inverters)for 30 min to 1 hour with dual batt's (to the stby DC and AC buses). Having flaps down at this point will be the least of your worries.

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