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737NG LED's and speed limitations/ops

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737NG LED's and speed limitations/ops

Old 16th Nov 2015, 10:00
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737NG LED's and speed limitations/ops

Hi all

Apologies if already covered, any references appreciated as know "the idea" but can't find more extensive explanations.

T/O in VNAV mode
Above acceleration altitude VNAV will set 230kt until all flaps (more importantly all LED's) are retracted as the LED's (apparently) have a limitation of 235 kts.
Reference??
Asking as ONLY limitation generally used is 250kts when flaps out (1, 2, 5 degrees)...full stop!

APP in MCP mode
Accept (and use) Boeing procedure as "set flaps up speed, approaching same select flaps 1..." etc etc however IN THEORY there appears no restriction in selecting flaps 1, 2 or 5 at 250kts and maintaining that same 250kts??
Any further flap selection of course at a speed of 210kts or less.

Asking as a F/O with flaps out after T/O selected SPD INT and set 240kts (scared of 250 below 10,000' which in Japan creates a paranoia so severe that it virtually requires the changing of flight deck seat covers a couple of times a day to remove the brown skidmarks!), I suggested 230kts initially and all OK.
Later when I tried to explain I couldn't - at least referring to the "simplistic" limitations we are provided where it is just "250kts EOFS"!
I'm aware of the 230kts limitation for alt extension but that's a non-normal.

I know there's a reason why the "250kts" is not the full story...but I don't understand "why".
Any assistance appreciated.

Cheers
galdian
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Old 16th Nov 2015, 11:21
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Above acceleration altitude VNAV will set 230kt until all flaps (more importantly all LED's) are retracted as the LED's (apparently) have a limitation of 235 kts.

Will it? I've never made a VNAV takeoff, so can not confirm. I thought the most common method was to BUG UP at acceleration altitude and then select VNAV when flaps are up & clean. To use VNAV on takeoff, as we did in B767, would perhaps require a V2+20 speed restriction unto acceleration altitude. In VNAV, once passing this altitude, the speed bug would increase. Is this the procedure you are referring to?
Why, on takeoff, would you want to select 240kts BEFORE clean. On even the shortest flight how many seconds is that going to save, if any? So why do it? Not good airmanship.

The 235kts is for LED's at FULL EXT & is used at the beginning of ALT EXT as you say.

On APP why would you select through to F5 and maintain 250kts? The only reason could be to increase ROC. That's what SPD BKS are for. The FCTM says Flaps should NOT be used as speed brakes, but I suspect that is more for the slowing down scenario. F5, SPD BK 210kts is a great 'get you down quick' configuration if held high and nearing the approach route.
Another safety reason for not maintaining high speed with LED's out is system stress. Limitations are not there to be used as the norm', (except if they are FTL's and minimum fuel.) Plus, what happens if there is an LED malfunction, or you mis-select F10 and the LED's go to FULL EXT at 250kts. You might lose an LED and end up in an UA.
If you are selecting flap on app then you are getting closer to the runway and you need to slow down, hence the flaps; so why fly high speed with lift & drag? Not good airmanship.

This desire to fly high speed is questionable. You program VNAV PTH to achieve an energy profile from TOD to OM. It usually includes a 250/FL100 transition. I used to fly with so many guns-ho macho wiz-kids who asked for high speed below FL100. ATC said OK. Later o the were pulling SPD BK's. Go figure. Not good airmanship. Same guys drive high speed towards red traffic lights and roundabouts. Not good chairmanship.
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Old 16th Nov 2015, 11:58
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VNAV takeoff (armed on the ground, active at) is the standard boeing SOP since FMC 8.1. And yes, it does command 230kias until the LEDs are retracted. From what i heard is the 20kts buffer simply a safety buffer to the 250kias limit for sloppy flying. Especially if the autopilot is already active and the plane encounters a windshear the ability of the autopilot to recover would not be sufficient if VNAV commands 250kias.
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Old 16th Nov 2015, 12:12
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VNAV will command 230 KTS until flap retraction in accordance with the skew protection speeds advised to prevent roll if the devices are asymmetrical. On take off we generally high increasing energy and higher angles of attack, on approach, energy is decreasing and lower angles of attack. It just makes for good airmanship.
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Old 16th Nov 2015, 12:56
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Thanks all for replies, however maybe some misunderstandings.

All have offered OPERATIONAL thoughts, I'm asking if anyone can advise WHERE:
- the alleged 235 kts limit because of LED's can be sourced;
- why, in simple terms it appears there's a "phantom" 235kt limitation on departure but, in theory, 250 kts for flap extensions 1,2,5 on approach are OK.

RAT 5

Thanks, but your first statement is that you "don't use VNAV" and bug up manually.
That's fine - but my question was related to VNAV which for myself has been SOP on the NG for the last 10 years.
My question regards APP (approach) well read again please, I follow the FCTM but the question was, IN THEORY you could select flaps at 250kts.

I am not asking about personal operational philosophy - just asking why there appears "hidden" speed limitations which (apparently) also vary whether on T/O or on APP.

If anyone can assist appreciated - just one of those "bugging me" things.
Cheers
galdian
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Old 16th Nov 2015, 21:53
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Originally Posted by Denti
VNAV takeoff (armed on the ground, active at) is the standard boeing SOP since FMC 8.1.
Not entirely standard, as there is an active bulletin on the matter which some airlines take to heart in not allowing crews to use the mode as advertised until a fix has been provided:

Subject: Impact of Arming VNAV on the Ground on the Windshear Escape Maneuver (FMC Update 10.8 and 10.8A)
Reason: This bulletin informs 737NG flight crews of the need to revise the windshear escape maneuver if VNAV has been armed on the ground for takeoff (FMC Update 10.8 and 10.8A).

Background Information
During airline simulator evaluations of FMC Update 10.8A, an unintended consequence of arming VNAV for takeoff was discovered with respect to the windshear escape maneuver. If VNAV is armed on the ground, it will remain armed even if TO/GA is pressed below 400 feet AFE in accordance with the windshear escape maneuver. At 400 feet AFE, VNAV will engage and windshear guidance will be lost. TO/GA must be pressed a second time when above 400 feet AFE in order to regain the appropriate windshear guidance.
This condition exists on FMC Update 10.7, 10.8 and 10.8A. Arming VNAV on the ground is not permitted prior to Update 10.8, however.
Boeing and GE are discussing a potential fix for this anomaly in a future FMC software update.

Operating Instructions
Arming VNAV on the ground is not permitted with FMC Update 10.7 or earlier.
With U10.8 or 10.8A installed, if unstable weather conditions are present prior to takeoff, consider not arming VNAV for takeoff.
If VNAV has been armed for takeoff and windshear is encountered, press either TO/GA switch and accomplish the Windshear Escape Maneuver. If windshear conditions continue above 400 feet AFE, press either TO/GA switch a second time when above 400 feet AFE. Verify TO/GA engagement and continue the Windshear Escape Maneuver.
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Old 17th Nov 2015, 01:27
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Wink

Well, I'm not sure if this is the reason, but:

Let's say that during your departure, after commencing the flap retraction, you get a LE FLAP TRANSIT.

Let's read the Flight Controls - LE FLAP TRANSIT NNC, shall we ?

Guess what (9.12 - Step 6) !?!

Light(s) for more than one leading edge device
is illuminated:

Limit airspeed to 230 knots maximum.


I don't know but, probably, Boeing is just trying to help us don't fck things even further in case of a LE devices malfunction. Max speed will be there, don't touch nothing, read the checklist, voilá.


Last edited by B737SFP; 17th Nov 2015 at 04:15.
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Old 17th Nov 2015, 04:20
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I've heard as many theories on the 230 kt VNAV bug as there are stars in the sky...

Rhetorical question... what is flaps up manoeuvring speed at MTOW?
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Old 17th Nov 2015, 09:01
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Rhetorical question... what is flaps up manoeuvring speed at MTOW?

There is a multitude of MTOW (structural) depending on how much you paid for. At 75.0T it is 221kts on NG-800
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Old 17th Nov 2015, 09:14
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Not entirely standard, as there is an active bulletin on the matter which some airlines take to heart in not allowing crews to use the mode as advertised until a fix has been provided:
Which is then a company SOP. In the bulletin you posted the base assumption is that VNAV take off is the standard and under certain conditions the pilots should consider to deviate from that standard. SOPs after all are only standard procedures, and pilots can, and often should, deviate from them.
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Old 17th Nov 2015, 12:06
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Thanks all

Appears no one can assist with original question - is there anywhere where Mr Boeing explains why VNAV sets 230kts until all flaps/LED's are fully retracted YET on approach (in theory) you could set flaps 1,2 or 5 at - and maintain - 250kts.

Cheers
galdian
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Old 17th Nov 2015, 13:04
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Just as a matter of curiosity...

Let's say that you guys are doing a normal takeoff with VNAV and LNAV engaged (FMC 10.8A+)... In case you have an engine failure, what your company SOP says ? Do you guys revert from Vnav to LVL CHG at the acceleration altitude, or do you guys fly VNAV even when single engine ?

My company says that we should request LVL CHG and set flaps up speed at the acceleration altitude, but, BOEING clearly says in the FCTM that when doing a VNAV TO, in case of an EO, VNAV will set FLAPS UP speed at the acceleration altitude...

So... I don't know if they missed that part, or if there's anything else that I'm not aware of.

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Old 17th Nov 2015, 13:06
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Galdian, if you are limiting your thinking process to VNAV constraints, VNAV will set the max speed for the alternate flap extension as 230 its due no skew or asymmetry protection. On approach in VNAV if you select flaps at 250kts the command speed will be the speed for the current flap section.. not 250 its. If you select flaps above placard speed the A/T lim amber light illuminates with msg " drag required" . Of course we can be num skulls and select flaps at any speed, but thats just poor airmanship, poor understanding of energy management and poor systems knowledge. Flaps are lift devices and not recommended as drag devices or to increase ROD (FCTM)
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Old 17th Nov 2015, 13:39
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The FMC should recognize a OEI situation and then switch automatically into the OEI mode, using the OEI acceleration altitude and speeds. Therefore VNAV is not a bad situation if you fill in the take off page 2.
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Old 17th Nov 2015, 13:59
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There is a multitude of MTOW (structural) depending on how much you paid for. At 75.0T it is 221kts on NG-800
Yes, and at higher MTOW it gets closer to 230kts. Ours are 79.0T and it's 224kts. I'm just throwing in another theory that was suggested to me once upon a time... Plausible, but with no more authority than any other theory I've heard.

Some suggest that it's due to a LED limitation of 235kts. That is not published in my manuals. The LED limitation is obviously 250kts for flap settings up to 5 (and alternate flap extension is irrelevent to this discussion).

However, up to flaps 5, the LEDs are not fully deployed. Boeing permits up to flaps 25 for take-off. In non-SFP variants, the LEDs are fully deployed at take-off with flaps 15 or 25.

So, is there indeed a secret 235kt limitation on fully deployed LEDs?

During a VNAV flaps 25 take-off, does it still bug 230kts? If so, it might be protecting the LEDs, but it ain't protecting the trailing edge flaps. (I'm asking because I don't know. I haven't ever tried greater than flaps 5 for VNAV take-off.)

Another question is: why a hard 230kt figure in the first place? Lazy programming? VNAV knows the flaps up manouevring speed for the aircraft weight, so why doesn't it just bug that? After all, that is what gets bugged via the MCP when flying non-VNAV departures.
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Old 17th Nov 2015, 20:13
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The QRH LE FLAPS TRANSIT NNC asks you to limit your speed to 230kts if more than one LED is extended and TE flaps are retracted.

It is also the limit for use of Alternate Flaps.

235kts is the gear retract limit speed but nothing to do with flaps.
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Old 18th Nov 2015, 02:28
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The QRH LE FLAPS TRANSIT NNC asks you to limit your speed to 230kts if more than one LED is extended and TE flaps are retracted.
And that is relevant how, exactly?
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Old 18th Nov 2015, 05:59
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I see the relevance. I have never had it happen apart from in the sim but if I am cleaning up and I get the leading edge flap transit light, the first thing I plan on doing after establishing who is flying the aircraft and that terrain isn't a concern is selecting v/s 1500 and 220knots and asking for the checklist. The vnav function of limiting to 230 will have kept me within limits until that point.
Fair?
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Old 18th Nov 2015, 12:26
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Originally Posted by galdian
is there anywhere where Mr Boeing explains why VNAV sets 230kts until all flaps/LED's are fully retracted YET on approach (in theory) you could set flaps 1,2 or 5 at - and maintain - 250kts.
It has to do with the limitation of LED when fully deployed.
On most takeoffs (using Flap 1 or 5) and when selecting 1-5 on approach the are partially extended and have a 250kts limitation. Fully extended the limit is reduced.

VNAV helps pilots in many ways, as described by others above, like in OEI operations provided Takeoff page 2/2 is entered with relevant data (acceleration altitude for NADP 1 or 2, OEI retraction altitude etc), or 240kts below FL100 in descend to allow buffer not to exceed 250kts (many of out company pilots seem to believe this is due to a FMC coding error, or requirements in USA, but this is not the case (ref: Boeing Supplementary FMC Documents regarding VNAV algorithms)), it prevents overspeed in both Mach and KIAS flying range, equally will not allow exceeding 230kts when LEDs are extended during flap retraction.

The guys and gals that wrote the software for VNAV are very bright and not all their decisions are described in our manuals. But lets just summarise it in the fact that VNAV under no known condition will (autonomously) take the aircraft outside its protected range, or else it would not be certified by FAA without limitations.

And framer: yes VNAV would have kept you safe, no need for LVL CHG or V/S
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Old 18th Nov 2015, 20:23
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Skyjob
OK hopefully the last question:


- on descent NOT in Vnav but MCP mode we can - in theory - set flaps 1, 2 or 5 at 250kts and maintain 250kts.


There is no limitation or mention made by Boeing in this case.


On takeoff in either Vnav or manually setting green dot we have 230kt protection - understood.
On descent in MCP we - in theory, as above - do NOT have any note or limitation.


Why are the LED's so important on takeoff but - potentially, in theory - not on approach??


I am well aware of recommended not to use flaps as speedbrakes/the alternate flap extension of 230kts etc etc which are side issues to the question.
Cheers!

Last edited by galdian; 18th Nov 2015 at 20:24. Reason: spelling typos.
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