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Questions about V1

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Questions about V1

Old 23rd May 2020, 12:37
  #21 (permalink)  
Join Date: Aug 2007
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Originally Posted by safetypee View Post
8che, et al, "More flap ?? on the take off roll in wind shear ?"
Precautions on take off are discussed on page 38-39 of the FAA reference #14.
Re flap, 'more flap provided greater performance for WS encounters on the runway, but lesser flap settings provided better performance for in-the-air WS encounters'.
An aircraft and situation dependent compromise - resolved by delaying departure.

Re "…so do what you feel is the best you can on the day", this is a judgement based on knowledge, experience, training, guidance, etc, which must be balanced with a healthy respect of 'fear'.
This can be described as airmanship or common sense, neither of which has universal meaning, interpretation, or application. The best on the day is safe, again delay departure.

Culture, etc. A reoccurring issue; in a perfect world we would not be fired, or if imperfect, we continue working for the wrong company - needs must.
Thus in the real world, pilots' require the judgement of Solomon and the skills of a 'Nelsonian eye'.

With an intolerant operator, for wind-shear conditions, protracted start and taxi procedures, or if pressured at the runway - a 'fictitious', intermittent, cargo door warning (the sort that an officious P2 doesn't see.)
Return to stand, have the door checked - by P2, outside in the wind, rain, and thunder.

Then seek views on wether the runway might be flooded - s/he would have to go outside agin for a 'puddle stomp test', or place 2 cents on the ground to judge water depth.

Wind-shear conditions and a flooded, limited distance, runway; a most likely combination for cb activity.

Thats 2 cents of experience; two people making sense of the situation.
SafetyPee, read the conversation again. More flap BEFORE the take off roll commences great. Changing it in the middle of the roll is not something the FAA or Boeing would ever endorse..
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Old 23rd May 2020, 16:08
  #22 (permalink)  
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Originally Posted by 8che View Post
SafetyPee, read the conversation again. More flap BEFORE the take off roll commences great. Changing it in the middle of the roll is not something the FAA or Boeing would ever endorse..
Lets be clear what we are talking about here; you are barrelling down the RW with, for example, a V1 of 130 kts (I'm in an A320). Shortly after the "100 kts" call - say for arguments sake 110kts - the speed trend arrow goes from the top of the speed scale to zero and the CAS stagnates. The wind arrow on the PFD shows a significant and unforecast tailwind.The end of the RW approaches - whats to do? Clearly, your calculated performance is now out of the window and you are in survival mode - the FCTM/FCOM no longer helps.
TOGA - sure.
Rotate at 110kts - maybe.
Sit and wait for Vr - could be noisy and expensive - GS is by now way higher than the CAS
If an increased flap setting is available to aid the ac to fly - maybe.
Stop - no way.

Other positive contributions are of course welcome.
mcdhu is online now  
Old 23rd May 2020, 17:15
  #23 (permalink)  
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The modelling representation of windshears in simulators has the same basis as that of the aircraft - measured data. But while the aircraft model is derived from aircraft data, the windshears (and indeed other things) are dervived from more general data. There are in fact a range of "canned" wind profiles derived from a number of real life events, and those can be used (they are also used in windshear warning/guidance certification tests).

I would be very cautious in assuming that the assumptions on which the aircraft performance calculations are based are themselves regulatory, especially in the operational world - just because Part 25 says to assume something doesn't mean part 121 (say) enforces it. I would also be very cautious about extrapolating those same performance assumptions outside their intended scope; there's nothing in part 25 about assuming any kind of windshear for the purpose of performance calculations - the closest there is would be the 0.5x/1.5x factor on STEADY wind, but that's hardly applicable to the shear case.
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Old 24th May 2020, 02:09
  #24 (permalink)  
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It might be appropriate to keep in mind that the design and operational rules, inter alia, are based on rational statistical history relating to accidents and incidents

The rules (and, by inference, the SOPs which, largely, are based on the rules) do not provide anything like an every day guarantee that an aircraft heading off to the holding point will, in fact, successfully find its way to the other end of the flight planned for the day. History certainly indicates that the rules get it right a very high proportion of the time so they should be viewed as being not too bad overall - but not infallibly correct and appropriate.

Even with the bit of padding and fudge factoring which is embodied in the rules, if circumstances on the day get a little too far away from the presumed story, then one has to accept that, on such days, one really ought not to have got out of bed. On flights for which such applies, the rules don't really give us much comfort and the captain, yet again, finds that when he (she) looks to the left to seek guidance, there is only the well-known face reflected in the side window - once again, the captain realises that he (she) can be in a very lonely place. Sometimes, the captain is called on to take decisions for which some of the rationale can only be guesswork - that's the nature of command, unfortunately.

Windshear is one of those considerations for which the rules can be found wanting. If a pilot cannot accept the reality that flying is not a perfect activity and that risk cannot be regulated totally out of the equation then, perhaps, that pilot ought to seek alternative employment.

As a side comment, knowing who folks such as safetypee and mad (flt) scientist are, might I suggest that their (very experienced and competent) technical/operational expertise warrants that their counsel be considered ...

I think safetypee summed it up, if I might be permitted to quote - "The critical decision in potential wind-shear conditions is in the choice to takeoff, opposed to delay."
john_tullamarine is offline  
Old 24th May 2020, 03:15
  #25 (permalink)  
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Originally Posted by 8che View Post
Fair enough but its not an option and a dangerous suggestion so I have to call it out. The number one rule of wind shear recoveries is don’t change the config. Please don’t believe everything you see in a simulator. While a level D sim will almost certainly contain the actual fight modelled data of an airframe that is or was in service, the modelling of external environmental conditions is a pure manufactured mathematical model and so can only ever be preprogrammed. It can never therefore accurately model real world environmental and aircraft response. In short it ain’t and never will be the real aircraft response, just the best simulation.
To supplement the above. Windshear scenarios are typically based upon the FAA Windshear Training Aids (WTA) developed at the end of the last century and Microbursts are typically based upon either the NASA TM 85969 or the Royal Aircraft Establishment (RAE) models again from the last century. These are very much training tools/aids and generic in nature.
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Old 24th May 2020, 04:38
  #26 (permalink)  
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I agree with John Tullamarine.It is impossible for SOPs to cover every eventuality. There are times when they are not appropriate to the situation and the captain must decide to deviate from them. This is an extreme measure and should only be done when there is no other option and when the pilot has sufficient knowledge to understand fully the implications of what he/she is doing. This is what command means. I can think of several examples.

Suppose on take-off at maximum weight in a four engine aircraft (a 747 with multiple landing gears) at a hot and high airfield, birds are ingested into both engines on the same side. Then, when airborne only a few feet above the ground, and barely able to climb, the co-pilot calls positive rate of climb, but the captain elects to leave the gear down until more speed and altitude has been gained. Non-standard, but his reasoning is that the extra drag of the open doors and wheel wells may well cause the aircraft to sink back towards the ground.

Suppose on final approach all engines suddenly stop. The captain elects to retract the flaps one notch. Non-standard, but the drag reduction is just sufficient to stretch the glide and make the airfield.

Suppose in mid-Atlantic, the underfloor cargo fire warning lights illuminate, the crew have reason to believe the warning is genuine and divert to the Azores. When they arrive the weather has deteriorated, the cross-wind has risen above limits and the cloud base has descended to below decision altitude. There is nowhere else to go and it is imperative to land. They do so successfully. All three examples were genuine events. It is not difficult to dream up other scenarios where such actions may be necessary.

And one more to ponder: the Qantas Airbus A380 at Singapore when the turbine disc exploded causing extensive damage and multiple unrelated warnings. In what order should the crew action the various procedures, which should be ignored and which have to be actioned?

Any serious captain should study his/her profession sufficiently so that he/she is prepared for such extreme eventualities.
Bergerie1 is offline  
Old 24th May 2020, 09:16
  #27 (permalink)  
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Check your documents for some wording. My company (and every one I have worked for) has something in the company documentation that roughly says:

“There cannot be a procedure for every scenario and nothing in these documents prevents the PIC from taking whatever action is necessary for the safe outcome of the flight”.
AmarokGTI is offline  
Old 24th May 2020, 09:27
  #28 (permalink)  
Join Date: Mar 2006
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Slight thread drift - into the air; re ZFT's comments on simulated wind shear models.
The serious incident (accident) reported and discussed in the link below shows the correlation of a real-world wind shear with a simulator training model. As I recall the origin of the FAA model 4 was the Dallas accident.
The report also enhances JT's and Bergerie's points about 'captaincy', 'Captains In Command'.


PEI_3721 is online now  
Old 24th May 2020, 10:53
  #29 (permalink)  
Join Date: Sep 2004
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As highlighted above, you need to exercise your airmanship.

I use to operate a Global from a 15000ft runway. You could take-off at maum (100,000lbs), positive rate, gear up, climb to 100ft, change your mind, gear down, Flap 30, land ahead stopping before the end. Yet, on the initial take-off run at V1 and above you are mandated to go - and would probably be criticised for aborting above it. You have to know your perf and understand exactly what and why it is telling you!
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