smartt1982
Topic Author
Posts: 226
Joined: Sat Nov 24, 2007 11:17 pm

737 LE Devices + Flap 15 Landings

Sat Jun 23, 2012 5:07 pm

Hoping someone can shed some light/info on a few questions I have about the Boeing 737 specifically in regards to the Flaps system and also the reason for Flap 15 Landings for a number of Non Normal situations.

In regards to the leading Edge devices, is it a coincidence that the limit speed for the full extension of the LE Devices (230 Knots) is the same as the alternate flap extension speed of the TE Devices?
Why is it that if you only have one LE Device out (in a non-normal for instance) your limit speed is 300 Knots/.65 Mach but any more than one it reverts to the 230 Knots.

The other questions relates to the reason why the selection of Flap 15 is dictated so much in a number of non normal situations.

For instance I can understand (or think I do!) why Flap 15 would be best suited for landing with:
- Loss of SYS B, it would take too long in the case of a Go-around to retract the flaps electronically hence degrading your go around performance
- Jammed stabiliser, you possibly would not have enough elevator authority to round out/flare with greater flap setting
What I cannot seem to understand or figure out, is the most obvious reason why it is scripted to Land Flap 15 in the following non normal situations.
-LE FLAPS TRANSIT light stays illuminated after the flaps are up.

-Jammed or Restricted Flight Controls

-Trailing Edge Flap Disagree Cx List with the option of selection: Indicated flap position is 15 or greater and less than 30: Land using existing flaps.

In relation to these why Flap 15, particular on the last example why not land with Flap 25 (lower landing sped etc).

In relation to other Boeing aircraft, is there a particular Flap setting that is scripted in the majority of situations, what about Airbus etc?

Any info would be greatly appreciated.

Thanks
 
tdscanuck
Posts: 8572
Joined: Wed Jan 11, 2006 7:25 am

RE: 737 LE Devices + Flap 15 Landings

Sat Jun 23, 2012 5:23 pm

Quoting smartt1982 (Thread starter):
In regards to the leading Edge devices, is it a coincidence that the limit speed for the full extension of the LE Devices (230 Knots) is the same as the alternate flap extension speed of the TE Devices?

The limit speeds are often structural limits, so it's probably not a coincidence.

Quoting smartt1982 (Thread starter):
Why is it that if you only have one LE Device out (in a non-normal for instance) your limit speed is 300 Knots/.65 Mach but any more than one it reverts to the 230 Knots.

You basically have to be able to handle any single failure (e.g. one LE device out) at any time; the chances of more than one happening independently are very very small so you can shrink the envelope down for multiple failures. Otherwise you'd have to build the structure to handle multiple LE devices out at 300 knots.

Quoting smartt1982 (Thread starter):
What I cannot seem to understand or figure out, is the most obvious reason why it is scripted to Land Flap 15 in the following non normal situations.

In many cases, you have ambiguous aerodynamic configuration so you don't know your true stall speeds. Flaps 15 drives you to a higher landing speed, which increases your stall margin. It's basically an easy and "flight crew friendly" way to inject additional performance margin. You also can't guarantee single-engine go-around performance at deep landing flap settings.

Quoting smartt1982 (Thread starter):
In relation to these why Flap 15, particular on the last example why not land with Flap 25 (lower landing sped etc).

With a trailing edge flap disagree you don't know where the flaps are, exactly, and you don't know if they'll move when you try to retrace them if you end up in a single-engine go around. With landing flaps and unknown ability to retract you can't show you can make the required gradient on the climb-out.

Quoting smartt1982 (Thread starter):
In relation to other Boeing aircraft, is there a particular Flap setting that is scripted in the majority of situations

For the 777/787 it's almost always Flaps 20.

Tom.
 
smartt1982
Topic Author
Posts: 226
Joined: Sat Nov 24, 2007 11:17 pm

RE: 737 LE Devices + Flap 15 Landings

Sat Jun 23, 2012 7:13 pm

Cheers, thanks again Tom
 
737tdi
Posts: 1116
Joined: Sun Sep 02, 2007 10:05 am

RE: 737 LE Devices + Flap 15 Landings

Mon Jun 25, 2012 8:16 am

Quoting smartt1982 (Reply 2):
Cheers, thanks again Tom




To make you a little more comfortable and reinforce what Tom said, I have been working on 737's for over 20 years and aircraft for over 30, and have never heard of a dual slat/Kruger flap failure ever. Sure, I've seen a slat fail to retract or a kflap fail to retract/deploy but never multiples, unless the flap/slat control valve fails then you lower your aft. flaps electrically and don't worry about the slats. Just change the profile. By the way 99.9999 percent of flap failures are indication. The flaps/slats are working fine, the prox. switches/reed switches have failed. That's why you have MEL relief on the slats.

By the way, it's the same for fire/overheat detection, it's 99.9999 percent that it's the detector. You can't take a chance with that though. Fire warning, get on the ground now. Chances are I will come out and change the detector and all is good again. Sure wish they could be more reliable, or maybe not. I need the overtime!!! Just kidding.

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