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B738 LE Flaps&Slats ALT EXTENSION

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B738 LE Flaps&Slats ALT EXTENSION

Old 16th Mar 2018, 06:38
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B738 LE Flaps&Slats ALT EXTENSION

Does anybody know where to find any guidance on the time it takes to extend the LE devices using the ALT extension switch, as directed by the Trailing Edge Flaps Up Landing NNC?

I had this scenario in the sim and after momentarily moving the switch to the down position I continued to monitor the LE annunciator panel for a good 10 seconds and nothing happened. Consequently we configured for an ALL FLAPS UP LANDING however the LE devices had eventually moved to the full ext position and we obviously missed it.

I couldn't find any reference to the time it takes for the LE devices to extend in this scenario. Any thoughts?
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Old 16th Mar 2018, 09:17
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Leading Edge devices full extension with ALT FLAP EXT will take 1 minute. Start your Chrono, first 30 seconds you will see the LE flaps extended, end of 1 minute you will see the LE slats also full extended (see this on the LE DEVICES ANNUNCIATOR PANEL - AFT OVHD)

If this does not happen by the end of 1 minute. You can go ahead with ALL FLAPS UP LANDING NNC
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Old 16th Mar 2018, 09:51
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Not a specific reference but the answer you seek is approximately 2 minutes.
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Old 16th Mar 2018, 09:55
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Originally Posted by Chesty Morgan
Not a specific reference but the answer you seek is approximately 2 minutes.
He is not asking about the TE flap extension to flaps 15 (which takes 2 minutes). He is asking about how long leading edge flap extension will take
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Old 16th Mar 2018, 10:47
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The 2 mins mentioned is to reach F15. In your sim scenario it seems you were a little impatient. Perhaps understandable. It depends on what session you were doing. If it was a TR course then it seems unfair not to guide you; if it was a recurrency session then they were letting you make the mistake, but you must have forgotten this from your initial TR training.

The reason the LEDS take a long time is that they are using the low power SBY pump. In the sim they do take a long time to kick off, but I'd expect you to see the first Amber light within 10-15 secs; guessing. The LED's then go all the way to Full EXT and take a long time and also the TE's take a long time to reach F1 because the SBY pump is driving all of them. This can take 50-60 secs. Moving from F1-F5 is slow, but not overly as only the TE's are moving. The same from F5-F10 & F10-F15. The time for those is as normal HYD B speed.
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Old 16th Mar 2018, 14:29
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With the alternate flaps switch in the ARM position, the standby system is used to extend the leading edge devices. When you momentarily press the switch to down, the leading edge flaps and slats will extend to the FULL EXTEND position.

So you are using one small pump (standby pump) instead of one very powerful one (EDP) and another quite powerful one (EMDP) and you are extending the slats even further (FULL EXTEND instead of EXTEND), so things are bound to take more time than with normal extension.

I recall a ballpark figure of 30 seconds for LE flaps and 60 seconds for LE slats.
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Old 16th Mar 2018, 16:49
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Originally Posted by RAT 5
and also the TE's take a long time to reach F1 because the SBY pump is driving all of them.
The TE flaps are extended by an electric motor.
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Old 16th Mar 2018, 19:24
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Sonicbum: Quite correct. An embarrassing senior moment.
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Old 16th Mar 2018, 19:44
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Momentary,,,

Select and hold for ca 1 sec.
I do recall some issues on some sims with regards to this. Some students did a micro second and claimed it did not work.

I then did a quick second ( momentary) and it worked.
It then takes ca 1 min from up to full extend were you have the transit light on until full extend.

I also have done this Live in Hollywood with TE Flap asymmetry frozen at less then 1 and it went from LE Extend to Full extend in less then 30 sec. ( 20? )
With a firm "Momentary" selection of Alt flap switch to activate the stby sys.

Drop the gear ca 8 miles out , get down to Vref +5 asap as there is next to no drag. Ca 3,5 NU 174kts over the fence Autobrake max; ca 1300 meters.

A rather lengthy CL with a lot of crew ending up with a All Flaps Up landing due to misinterpretation or execution.

Good fun in the winter!
Happy landings
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Old 17th Mar 2018, 08:32
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I do recall some issues on some sims with regards to this. Some students did a micro second and claimed it did not work.

When you activate the ALT Flaps switch you are starting the SBY HYD pump. If it has been activated you should see the SBY HYD Oil Press light come on and go out. This confirms the pump is running. If it doesn't then it ain't, or the bulbs are broken. There are some later customer options with an extra light saying SBY PUMP.
Blipping switches, e.g. the APU start switch, is not a good idea. Positive action and follow up with other indication to confirm correct activation if possible is a good idea.
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