737 alternate flaps

[eng1170]

I was explaining to a trainee last week the operation of the alt. system on the 737-400. I explained that if no A or B system hyd's were available a down selection on alt uses the stby system to put the leading edges to FULL extend and the flaps to what ever is selected by holding the switch in the down position. I explained that selecting them up only retracts the trailing edge flaps and that until B system is restored the leading edges will remain fully extended.

Firstly is my understanding correct? And secondly (this is where I need a flight crew answer!!) the trainee I was talking to asked what happens regarding the leading edges if the crew then had to carry out a "go-around" with no A or B hydraulics?

My answer to his question was that if the situation was so bad that A and B system had been lost the crew would land the aircraft, and that the possibility of a go around would be unlikely as the crew would probably have declared a full emergency.

Is my analysys of the scenario accurate?

Many thanks in advance - Eng :confused:

[Herod]

Half right. The leading edge does indeed extend using the standby system, but the trailing edge is controlled electrically, hence the ability to retract them again. In the event of a go-around, the leading edge stays extended, with the consequent speed restriction. There is also the consideration that in the event of a diversion, there may be insufficient fuel to reach the alternate.

[Yarpy]

Your technical understanding is correct.

The procedure calls for a Flap 15 landing.

In the event of a Go-Around the slats remain at the full extend position.


You may raise the trailing edge flaps using the alternate system and woulh have to allow for the slower flap travel speed whilst accelerating. The setting you use would depend on your intentions, the structural limits and the minimum manouvre speed.

As regards the scenario you are a bit astray as you cannot stop sh*t happeneing. You might HAVE to do a Go-around depending on the circumstances. E.g. blocked runway, lack of visual refernence etc.

Loss of SYS A & SYS B would be a full emergency, of course.

[eng1170]

Sorry if it was not clear, I am aware t/e are operated by elex motor, (it's not that big for what it does incidentally!!) and that the T/E could be moved if reqd.

The only other thing that no one has mentioned is that obviously with both A and B systems out the gear would have been dropped manually and there would be no bringing it back up if a go-around was needed.

What sort of speed would you be looking at for a circuit in a -400 where the config is slats FULL EXTEND, FLAPS 15, GEAR DOWN, if you had to go-around, and how quickly would the aircraft respond to this, I'm assuming you couldn't just firewall the throttles and I was wondering if you'd have a much higher decision height given the lack of A and B hydraulics.

Excuse my ignorance, I'm a Licensed Eng, and am always fascinated by the flying side of thing's, understanding how you guy's deal with these scenario's can make it easier for me to explain to newbie's why we have these systems/how they work, and there limits.

Thanks again for the good answers,

Eng

[BOAC]

eng - most people would probably KISS and fly standard F15 speed, ie Top bug/150kts (160 if heavy).

Actual initial part of G/A not affected by full extend on L/E and gear, it is climb gradient that suffers. Probably no worse than s/engined though.

In aviation it is customary not to PLAN for double emergencies so you have to hope both donks keep running as well:eek:

[738]

If you go around in this config you would retract the flaps to 1 electrically.

On the technical side.

The L/E will go to full extend once the alt sw is selected on and the first selection of the toggle sw is placed to down...even for a second. They will stay full ext until the alt switch (guarded one) is place back to normal and norm hyd press is restored.

You are limited to 230 kts with L/E full ext so it would make sense to do the circuit just above clean speed...say 210kts

There would be no change in decision height but you couln't do a lo-vis landing (less than 200ft DH) because you need the automatics and they wont work without hyd.

There is plenty of power with 2 engines. It will climb at flaps 40 with 2 engines...but as already said you will not make the legal climb gradient required for a missed approach. Normally we have to configure the aircraft so it will climb at leat 2.4 degrees gradient.

Hope this helps