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744 Eng Fail and Vmcg

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744 Eng Fail and Vmcg

Old 1st Jan 2021, 21:43
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744 Eng Fail and Vmcg

Should you add forward pressure onto the nose wheel with the elevator at V1 (Vmcg =V1) to ensure centreline tracking? No mention of it in any FCTM but in some sims Iv seen you will depart the runway if you donít.
What Iím really asking is the definition of Vmcg , and does it mean increase pressure on the nose wheel if needed ( as it is Vmcg) and since itís not written any where , should we be doing it to stay on the runway or re-evaluate our V spds.
Any one else seen this?
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Old 2nd Jan 2021, 11:24
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Vmcg is defined as the minimum speed, whilst on the ground, that directional control can be maintained, using only aerodynamic controls, with one engine inoperative (critical engine on two engine airplanes) and takeoff power applied on the other engine(s). So where does the nose wheel come in?
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Old 2nd Jan 2021, 11:34
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The basic requirements for minimum conditions (which appears to be your concern) -

(a) Vmcg can't be less than Vef, rather than V1.

(b) so far as the NW load and NWS is concerned, the certification testing avoids both.

(c) CG max aft and, usually, minimum weight.

(d) XW nil

The problem is that, if you are playing in the real world near Vmcg, generally you aren't going to be replicating the certification requirements so it gets a bit rubbery. Be very aware that the directional control animal gets very aggressive, VERY rapidly, in the last few knots as the speed reduces toward Vmcg - the certification Vmcg flight test stuff potentially is REALLY hazardous.

A significant adverse problem is crosswind. If from the failed side of the house, the "real world Vmcg" (as opposed to the book figure) will go up. This delta can be quite significant and, for a 744, you might be looking at something in excess of 1kt/kt. In the case of a stiff crosswind, a takeoff with a planned min V1 schedule is something which one probably should try and avoid unless the V1 can be increased by an appropriate margin to avoid the XW concerns.

So, the short answer, generally, is that you ought not need to push the stick forward, but it is going to help the situation for the real world pilot, so it fits the bill as a "good idea". Forget centreline tracking ... that just isn't going to happen. I have archived video of some DC9 work we did (TP is a PPRuNer) and, as the runway head cameraman, it was quite illustrative to see just how the DC9 (said by just about every DC9 pilot to be an aircraft on rails with a failure) promptly leaped out of the viewfinder ... except for the wingtip. The aircraft will yaw, deviate and then, if things go well, either parallel or, maybe, come back towards the centreline a tad, prior to lift off. There is no specific requirement for either of these to occur and one could well see the deviation limited by the liftoff. Generally, the continued takeoff is the critical situation as, in a reject case, the throttles will be closed in short order.

Have a read of the flight test things at page 111 of AC 25-7C Flight Test Guide For Certification Of Transport Category Airplanes (faa.gov)

The current rulebook requirement is at FAR 25.149(e) - see Electronic Code of Federal Regulations (eCFR)

You would need to do some homework to see if the 744 met these requirements or something earlier and, perhaps, a little different. I'll leave that for your spare time ...
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Old 2nd Jan 2021, 12:04
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flyhigh55,
Alway use the procedures in the manufacturers FCOM.
Do not invent new techniques just to fly the simulator; simulators are good, but they are not the real aircraft.
Check the procedure being used, check the simulated runway conditions; all else being equal on a dry runway it should be possible to limit the centreline deviation to 30ft with prompt application of full rudder.

N.B. Forward stick in some aircraft can reduce main-wheel loading, if so, tendency to go sideways !

V1 is always greater than Vmcg
"Understanding Takeoff Speeds"
https://skybrary.aero/bookshelf/books/493.pdf

Airbus ďGetting to Grips with Aircraft Performance"
https://www.skybrary.aero/bookshelf/books/2263.pdf
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Old 2nd Jan 2021, 14:38
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Should you add forward pressure onto the nose wheel with the elevator at V1 (Vmcg =V1) to ensure centreline tracking?
No! The nose-wheel steering is likely disconnected at this point (don't know how it works on the 747) and it's definitely not designed to have the lever moment generated by the elevator at high speeds loaded onto it.
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Old 2nd Jan 2021, 15:19
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Good discussion. But as suggested above, there are no mandatory rules that govern the pilot's need for action under this condition. It's considered a remote surprise to the pilot requiring different judgements.
e.g.. imagine the condition where an engine goes to unwanted max power on one side at this speed condition. That will take several seconds to sort out
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Old 2nd Jan 2021, 22:22
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vilas- on the the 744 you would see that you do get better center-line tracking at Vmcg if you add pressure to the nose wheel using " aerodynamic forces- (elevator)" {not NWS)

Thank you John tullamarine. It is that gray area where X-Wind comes in that I have experienced the issues. definitely something to be aware of even if we are talking about a very small window.
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Old 2nd Jan 2021, 23:36
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No! The nose-wheel steering is likely disconnected at this point (don't know how it works on the 747)

Now, that's something I haven't come across before. While the FT folk might do their tests with NWS physically disconnected to tick the certification box, the idea of such happening out there on the line is a new one. Are you able to add some commentary as to any Types which might do such things, etc ?

it's definitely not designed to have the lever moment generated by the elevator at high speeds loaded onto it.

Likewise, might you be able to provide some evidence to support this contention ?

on the the 744 you would see that you do get better center-line tracking at Vmcg

That probably will be the case for the great majority of aircraft - anything which gives you some cornering forces is going to help. However, it is pertinent to keep very much in mind that the aircraft is going to diverge, regardless. Considering those test chart results I have seen, this applies even at high speed as there is always going to be bit of a delay before the pilot gets the rudder in and the aircraft responds. Generally, you are going to see a small deviation which starts to increase as you get back closer to Vmcg and then things really ramp up and get hairy in the last few knots.

It is that gray area where X-Wind comes in

I'd be a little more concerned than you, I suspect.

First, if there is much crosswind, all certification bets are off. Keep in mind that things like Vmcg are just lines in the sand for other certification activities. The pilot might not need to have a detailed certification knowledge but it sure helps to have an idea of which bits, and where, can bite you ... and badly.

The usual test requirement is nil wind. The old BCARs used 7 kt but we can discard that as being in the dinosaur realm. Consider that your four-motored beast might see something in the order of a, perhaps, 20-30 kt increase in the real world Vmcg with a stiff crosswind. That is to say, even allowing for the other conservatisms, you might find yourself considerably BELOW the real world on the day Vmcg but perfectly legal with your min speed schedule takeoff. The only problem is that, should a critical failure occur on the wrong side of the house, and you elect to continue, you ARE out of control and you WILL go for some tip-toeing through the tulips. In such a situation, V1 becomes irrelevant and the only option is a (slightly) above V1 reject.

Better, if you have the option, just to avoid the potential problem altogether. For example, the situation could well arise for an empty positioning flight from a long runway. If your laptop or RTOW book uses min speeds, then you would have the option to presume a higher weight and take advantage of the relevant higher weight speed schedule. We are just trying to get a suitable buffer to take us out of the nasty region.

Going back many years, I did a training contract on the 732. The operator had a routine positioning flight where this was absolutely relevant (and, often, windy). Once it became apparent that the folk had not thought much about this concern, some directed training quickly and clearly highlighted the problem and a suitable solution. Some of the wide eyed exercises were a delight to behold.

Although certification is pretty good and looks after us pretty well, that only applies most of time. Sometimes we need to read between the lines a tad.
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Old 3rd Jan 2021, 09:13
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Betwixt theory and practice, 'what-if', 'except', Ö reality.

Realism, an operational view.
NWS, fwd stick are theory, consideration of which risks creating inappropriate procedures, bad habits, biased mindset.

The practical rules are:-
#1 fly the aircraft as defined in the manual, as practiced.
Note that scenarios posed in debate are fixed, rarely like reality.
Adding distraction from a bang, vibration, fire bell, surprise, all consume mental ability - no 'what-if'. The minimum standard is to recall and fly the essential basics.
If you are at V1 you are going to fly; don't play with the stick, you will need to act at 'rotate' with back stick; how much, how different from the norm - recall training - can you describe it.

At V1 and above you GO; no redefinition of parameters or consideration of rarer 'except' situations. JT we need a word in the naughty corner; theory vs practice.

Crosswind, wet runways are real. Certification provides some margin - 30 ft and a second or two in dry conditions. In real-world adverse conditions the margins may be exceeded, using all runway width (how wide), even some of the grass; the piloting task is to fly as accurately as possible, avoid mishandling which could add deviation.

The 'what ifs' of these situations must be considered at pre flight briefing; a slippery runway, crosswind, and 'up north' snow banks, soft muddy ground.

Failures rarely occur at the critical point, but because of 'Sod' the worst case is practiced. Time doesn't stop as in debate or simulation, the aircraft will continue to accelerate, rudder increasingly effective, lateral excursions can be reduced.
Attention switches to getting airborne, and at lift off further 'surprise' from the forgotten crosswind - aircraft yaws, going sideways again.

Then how to avoid the obstacle which looks very close, fly V2; more distraction with increasing altitude - EGPWS (active >50 ft) sensing height over the real terrain (not in simulators), 'Don't Sink' - but you are already doing your best …

… and thats only the start of rule #1.
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Old 3rd Jan 2021, 09:33
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we need a word in the naughty corner; theory vs practice

Indeed.

My driving concern, though, is to have folks think about things in their armchairs over a port and at planning. For winds, if the crosswind is brisk and we might be inadvertently put into an unfortunate min schedule situation, can we mitigate the problem in advance so that the worry factor goes away ?

In the general situation what we train for is what we should do. Very rarely, though, circumstances conspire to defeat the theory game plan and we may need to be innovative to survive. Most of us aren't called upon to make those sorts of calls throughout our careers and, on the odd occasion, it just doesn't warrant getting out of bed in the morning.
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Old 3rd Jan 2021, 10:34
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Now, that's something I haven't come across before. While the FT folk might do their tests with NWS physically disconnected to tick the certification box, the idea of such happening out there on the line is a new one. Are you able to add some commentary as to any Types which might do such things, etc ?
Hi John,

The aircraft types I've flown (Airbus narrowbody, Boeing 787) both bleed out NWS orders from the rudder pedals as ground-speed increases. That's what I was implying when I said disconnected. My apologies, I could have made it clearer.

It's a moot point in my opinion because V1 is always greater than Vmcg, so if you can't control the aircraft with the nose wheels on the ground, you're going to face exactly the same difficulty during rotation.

Likewise, might you be able to provide some evidence to support this contention ?
Thinking it through, you're absolutely right. I suppose the elevator can only just overcome the weight of the aircraft on the nosewheel during rotation, and authority in the nose-down direction is probably less. Given the way the 787 falls onto its nose wheels during derotation it must be a non-event.

I don't know whether the elevator could generate sufficient force to wheelbarrow the aircraft onto the nose gear.
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Old 3rd Jan 2021, 10:44
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JT, I share your concern.
Tech Log is a very valuable resource; however the line between theory and practice is blurred, and quite often assumes that contributing posters have the same understanding.

Pilots need theory, but also and more important, how this is should be applied, the limits and assumptions in certification, and to consider the practical aspects, not least the self generated biases in risk assessment.

In our very safe industry there is increasing risk of self inflicted problems from weak understanding of the application of theory in practice. Humans like to consider the what-if situations, how to be a hero, save the day with innovative knowledge and procedure, whereas there would be greater benefit considering the 'everyday' situations which can be managed - before the event, avoided, mitigated (as below).

A testy, cold, snowy, and locked-down new year, dreaming of a sunbaked barbie (with port). Also the lack of the favourite Ozzy Red, replaced by an African white titled The Weatherman !

Situations involving lateral excursion which can be avoided.
https://safetyfirst.airbus.com/engin...ng-at-takeoff/
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Old 3rd Jan 2021, 10:58
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The aircraft types I've flown

Many thanks for that, good sir; yet, again, my knowledge base increases daily. I might speculate that the reason is associated with no need for NWS at higher speeds ?

It's a moot point in my opinion because V1 is always greater than Vmcg

And that is the basis of certification's trying to keep us out of harm's way by so arranging. Just that niggly real world crosswind problem which gives me some heart flutters at times.

I don't know whether the elevator could generate sufficient force to wheelbarrow the aircraft onto the nose gear.

Certainly some can but not the sort of thing one aims to do ( ... unless you fancy displays in Caribous - DHC caribou doing the "wheelbarrow" - YouTube - not quite the same thing but was always impressive to watch)

you're going to face exactly the same difficulty during rotation.

Although the extra speed might just be your saving grace.
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Old 3rd Jan 2021, 11:06
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I agree with safetypee:
Alway use the procedures in the manufacturers FCOM.
Do not invent new techniques just to fly the simulator; simulators are good, but they are not the real aircraft.
Inventing novel new techniques because you ‘know better’ than the manufacturer/operator/regulator leaves you very vulnerable, and not just in a legal sense.

For an engine failure after V1, the appropriate amount of rudder must be used to keep the aircraft ‘straight’ as the take off continues. If full rudder is insufficient to achieve that, how much should we then push the yoke forward? A bit? A lot? Full forward? Just before hauling the yoke all the way back to rotate? Sounds challenging to me.

That’s before we even consider the fact that we are going to be rolling down the runway at speeds well in excess of 100mph. We then load up two inflatable rubber tyres with a number of tonnes of aircraft mass and aerodynamic loading whilst they are not facing in the direction of travel?

Whilst this may not be modelled correctly in any simulator, (why on earth would it be?) I can reasonably imagine a likely outcome of the coefficient of friction in a ‘real world’ event could easily be two burst nosewheel tyres, before any noticeable turning moment occurs. Then what?

I’d prefer to be taught such novel techniques only after a test pilot (preferably the manufacturer’s!) has proved it’s a good idea. Rather than just because some bright spark thought it was a wizard wheeze!
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Old 3rd Jan 2021, 11:09
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however the line between theory and practice is blurred

Therein lies a major philosophical problem for us: what to do ? Do we dumb it down to the lowest common denominator and the whole thing becomes a PacMan game, only with real world hazard potential ? or do we throw out some information and robust discussion with a hope that most of the folks will contemplate such in the vein it is intended ?

Pilots need theory, but also and more important, how this is should be applied

Is not that paragraph the underlying crux of the whole educational problem ?

replaced by an African white

.. which brings to mind a delightful drop (Amarula) with which I became intimately acquainted during a SAA training contract long ago. Nectar of the Gods. Fortunately, it is imported to Oz so the affair can be maintained.

Hopefully folks will read and reflect on the Airbus note.
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Old 3rd Jan 2021, 11:12
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Inventing novel new techniques

I see no evidence of such in the thread. However, some reflection upon reality probably is of use to line folks.
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Old 3rd Jan 2021, 11:23
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From the very first lines of this thread:
Should you add forward pressure onto the nose wheel with the elevator at V1 (Vmcg =V1) to ensure centreline tracking? No mention of it in any FCTM
”No mention of it in any FCTM”

Sounds a pretty ‘novel’ technique to me.
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Old 3rd Jan 2021, 14:32
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I’d prefer to be taught such novel techniques only after a test pilot (preferably the manufacturer’s!) has proved it’s a good idea. Rather than just because some bright spark thought it was a wizard wheeze!
That's a thought to consider.

Hopefully there is some confidence from discussing stuff like in a tech thread to reduce the surprised factor when faced with an unfamiliar combination. The larger danger, however is to substitute such in place of what the manufactures have recommended.

The manufacturers spend lot's of time pouring over pilot incident reports to understand what works and how often. Of course they don't see the really rare stuff except one or two in ten years in a single design fleet.. The one thing that bothers me is listening to CVRs where pilots run out of options with words like "it's gotta work"
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Old 3rd Jan 2021, 14:33
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Originally Posted by john_tullamarine View Post
No! The nose-wheel steering is likely disconnected at this point (don't know how it works on the 747)

Now, that's something I haven't come across before. While the FT folk might do their tests with NWS physically disconnected to tick the certification box, the idea of such happening out there on the line is a new one. Are you able to add some commentary as to any Types which might do such things, etc ?

it's definitely not designed to have the lever moment generated by the elevator at high speeds loaded onto it.

Likewise, might you be able to provide some evidence to support this contention ?

on the the 744 you would see that you do get better center-line tracking at Vmcg

That probably will be the case for the great majority of aircraft - anything which gives you some cornering forces is going to help. However, it is pertinent to keep very much in mind that the aircraft is going to diverge, regardless. Considering those test chart results I have seen, this applies even at high speed as there is always going to be bit of a delay before the pilot gets the rudder in and the aircraft responds. Generally, you are going to see a small deviation which starts to increase as you get back closer to Vmcg and then things really ramp up and get hairy in the last few knots.

It is that gray area where X-Wind comes in

I'd be a little more concerned than you, I suspect.

First, if there is much crosswind, all certification bets are off. Keep in mind that things like Vmcg are just lines in the sand for other certification activities. The pilot might not need to have a detailed certification knowledge but it sure helps to have an idea of which bits, and where, can bite you ... and badly.

The usual test requirement is nil wind. The old BCARs used 7 kt but we can discard that as being in the dinosaur realm. Consider that your four-motored beast might see something in the order of a, perhaps, 20-30 kt increase in the real world Vmcg with a stiff crosswind. That is to say, even allowing for the other conservatisms, you might find yourself considerably BELOW the real world on the day Vmcg but perfectly legal with your min speed schedule takeoff. The only problem is that, should a critical failure occur on the wrong side of the house, and you elect to continue, you ARE out of control and you WILL go for some tip-toeing through the tulips. In such a situation, V1 becomes irrelevant and the only option is a (slightly) above V1 reject.

Better, if you have the option, just to avoid the potential problem altogether. For example, the situation could well arise for an empty positioning flight from a long runway. If your laptop or RTOW book uses min speeds, then you would have the option to presume a higher weight and take advantage of the relevant higher weight speed schedule. We are just trying to get a suitable buffer to take us out of the nasty region.

Going back many years, I did a training contract on the 732. The operator had a routine positioning flight where this was absolutely relevant (and, often, windy). Once it became apparent that the folk had not thought much about this concern, some directed training quickly and clearly highlighted the problem and a suitable solution. Some of the wide eyed exercises were a delight to behold.

Although certification is pretty good and looks after us pretty well, that only applies most of time. Sometimes we need to read between the lines a tad.
I don't remember anything about nosewheel steering becoming disconnected from rudder pedal input on the 747. That was on the Airbus if I remember correctly......at least the narrowbodies.

Even if the nosewheel does become disconnected on an aircraft type, it doesn't mean that more downward pressure on the nose won't help for directional stability(just like aft elevator helps for directional stability in the 3 point attitude on taildraggers). And, as I have discussed with John before(under a different handle of JammedStab), there is no consideration for crosswind in VMCG calculations, meaning actual VMCG can be much greater or much less than what would be published. It is one of those dirty little secrets that almost training organization/manual/etc never talks about. Then you get a surprise one off accident like the American Airlines A300 in NYC(where we discovered that Va is different than what many people thought it was) and word gets out.

Last edited by tcasblue; 4th Jan 2021 at 04:10.
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Old 3rd Jan 2021, 14:41
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Originally Posted by 4468 View Post
From the very first lines of this thread:

”No mention of it in any FCTM”

Sounds a pretty ‘novel’ technique to me.
There is also no mention in the FCTM that VMCG is invalid as soon as there is a crosswind. Even when we did one engine inoperative takeoff training in the sim on another 4 engine type I flew, no mention was ever made of the effect on crosswind for a situation where it really does have huge real world consideration for that takeoff. The winds were always calm for that scenario.
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