Using 'Conservative' Performance Numbers

[Airmann]

Was with a pilot the other day that wanted to do the performance numbers using a 5kt tailwaind even though winds reported were around 15kt headwind. Not the first time I've seen this but at the end we had numbers that were around 10-15kts shower than they should have been if we had used the real wind.

Just wondering if anyone could share their thoughts about the advantages or disadvantages of this 'conservatism'.

[giggitygiggity]

There's a million reasons why they might have done this. Experience might have dictated that your 15kt headwind regularly turns into a tailwind during the rotation. The winds could have been forecast to change so they went for the more conservative option. Could just be an element of airmanship to round down on the wind (although I admit that this is perhaps a way too far, 15kt headwind might reasonably be rounded down to 10kts to allow for a bit of variation in the w/v).

I assume in this case, it was probably just some habit they've developed, or perhaps they were trying to show off about how their enlightened conservationism was the superior method, when in reality it was perhaps not really a well considered request.

Did you not ask them why they wanted such conservative figures? Perhaps they had a good answer?

[Vessbot]

As a data point, at my company it's SOP to initially enter zero wind, if there's a headwind, for both TO and landing. (And then re-enter with a headwind if the zero numbers don't give required performance.) And it's recommended technique, if there's any tailwind, to enter a 10 knot direct tailwind (our max) in case there's a last-minute increase. (And, again, later walk it back if needed)

[ROW_BOT]

With the old Airbus performance manuals it was always an option for a Captain to reduce the Flex temperature if he so desired (yes, there are many good reasons why he might want to do so, and the book outlined the method).
But with the introduction of Flysmart on EFB most operators seem to have removed that option from Captains. They want Max Flex on every takeoff, no matter what the Captain might feel appropriate on a given day.
There is always the solution of going TOGA - but certain of these operators are known to ring up Captains and berate them for using TOGA. So, many invent other methods to avoid confrontation by avoiding using TOGA, but to reduce the Flex temp, and give themselves better margins, which won’t be noticed by management.
Adding tailwinds, doing performance figures from displaced thresholds, reducing runway length, using the wrong intersection - I’ve seen them all done, and I don’t like any of them. It’s fakery, and if anything at all goes wrong you’ll be hauled up to explain where you got your inventive ideas from, andyou will of course be hung out to dry.

The correct way is to go TOGA if you aren’t comfortable. That’s what I did, and I told the FOP Manager that’s what I’d be doing, and not to bother calling me for an explanation. They had left me with no other choice.
I was never harassed, but i know they took a severe dislike to me over it.

This is what our profession has been reduced to by the bean counters. Flying Monkeys, with less and less discretion every year that passes.

[Check Airman]

Absent some reason to doubt the reported wind (as noted above), using a number that's 20kts off is no longer conservative, it's wrong.

Correct me if I'm wrong, but don't the field performance numbers assume that the actual headwind is only half of what's reported, and the actual tailwind is twice what's reported? I could be thinking of something else though.

[Check Airman]

As a data point, at my company it's SOP to initially enter zero wind, if there's a headwind, for both TO and landing. (And then re-enter with a headwind if the zero numbers don't give required performance.) And it's recommended technique, if there's any tailwind, to enter a 10 knot direct tailwind (our max) in case there's a last-minute increase. (And, again, later walk it back if needed)

Not the way we do it, but it seems pretty reasonable. We do something similar for takeoff weight.

[john_tullamarine]

assume that the actual headwind is only half of what's reported, and the actual tailwind is twice what's reported?

half and one and a half rather than two.

I would want to know how the backroom chaps worked the wind for a runway with OEI turns and tiger country to contend with ... you might inadvertently be putting yourself into an undesirable turn radius situation ?

[compressor stall]

But with the introduction of Flysmart on EFB most operators seem to have removed that option from Captains.

This is a setting configurable by the EFB manager. He has the option in the setup to select 1. Show all results and highlight the flex or 2. Highlight TOGA and flex only.

What is worth remembering is that there might already be a flex pad put in place, which is a conservative buffer, on the perf data that Flysmart computes. An enlightened company might communicate this to the crews to prevent workarounds like the start of this thread. Or even better give the crew all the figures and let them decide on the day what is best.

[AviatoR21]

The real problem is when crew use actual conditions and then after seeing the windsock or having been given changed conditions do not recalculate. It is always better to use the conservative side. However, this case maybe a little extreme. I would have used nil headwind.

[Check Airman]

assume that the actual headwind is only half of what's reported, and the actual tailwind is twice what's reported?

half and one and a half rather than two.

I would want to know how the backroom chaps worked the wind for a runway with OEI turns and tiger country to contend with ... you might inadvertently be putting yourself into an undesirable turn radius situation ?
Thanks. That seems a bit more reasonable.

[Starfox64]

In my humble opinion, even taking nil wind for a reported 15kt HW seems a bit extreme. Remember performance already factors just 50% of HW component, yet 150% of a TW component. So your conservative calculation is already hugely taken care of before any pilot conservatism comes into play. Of course as mentioned above there may be local variances/experience to consider but overall all things being standard I wouldn't have an issue taking the reported HW.

The issue that is more common to face is extreme conservatism with T/O performance, only to see the aircraft doing 4000fpm into a low SID level off because OPT has been tricked into thinking full T/O power was needed. Much better to take an intersection figure, reduce T/O power and use full length. Plenty of margin and you won't be setting off anyones TCAS!

[Derfred]

What I have always wondered is this:

If the ATIS has 15kt headwind, and when I am cleared for takeoff the wind has reduced to 5kt headwind, will the tower tell me? If not I am taking off with invalid data.

[sonicbum]

What I have always wondered is this:

If the ATIS has 15kt headwind, and when I am cleared for takeoff the wind has reduced to 5kt headwind, will the tower tell me? If not I am taking off with invalid data.

No, they will simply tell you the wind direction/intensity as with every takeoff/landing clearance as they assume (and hope) You can work yourself out the head/tail/cross wind component.

[sonicbum]

Was with a pilot the other day that wanted to do the performance numbers using a 5kt tailwaind even though winds reported were around 15kt headwind. Not the first time I've seen this but at the end we had numbers that were around 10-15kts shower than they should have been if we had used the real wind.

Just wondering if anyone could share their thoughts about the advantages or disadvantages of this 'conservatism'.

The problem here is not the fact that he has elected to use some tail wind rather than headwind, but that he did not explain to You the reason behind this decision, which is a recurrent discussion/debriefing point in most training/checking events, even regarding small issues. The worst thing in a flight deck is taking a decision and leaving the other guy puzzled with question marks on his head.

[Derfred]

No, they will simply tell you the wind direction/intensity as with every takeoff/landing clearance as they assume (and hope) You can work yourself out the head/tail/cross wind component.

Maybe they do that where you fly.

[sonicbum]

Maybe they do that where you fly.

You mean You fly to controlled airports where You get a take off / landing clearance without any wind direction/intensity ?

[FlyingStone]

There is conservative and then there is behaviour limiting on anxiety. As stated above, headwinds/tailwinds are factored in the RTOW tables or performance software.

I would say ask the guy if he can explain the reasoning above, as you want to learn some command traits, but high chance you’ll get another unreasonable answer.

There is always the solution of going TOGA - but certain of these operators are known to ring up Captains and berate them for using TOGA.

Is this for real? I’d expect a ring if one day you have to offload half of the aircraft in 40kt headwind, because you were “conservative” and took 15kt tailwind and 30C higher OAT, but not for the use of TOGA.

With the old Airbus performance manuals it was always an option for a Captain to reduce the Flex temperature if he so desired (yes, there are many good reasons why he might want to do so, and the book outlined the method).

Interesting. The Boeing’s T/O thrust system is in my honest opinion much better and more flexible than the Airbus one, as you normally have 2-3 derates available and each of them can be combined with an assumed temperature vs just FLEX or TOGA on the Airbus (A320 at least), thus giving one many combinations to choose from, as appropriate on the day.

[Capt Scribble]

Our E performance gives the stop margin for a Flex calculation and shows the increased margin for decreased flex. I dont change it myself, and on short/narrow runways max flex is recommended. However, the aircraft perf will be better than predicted because the accel and stop distance and engine performance is assumed to be at the temp inserted as Flex, as the actual temp at take off is lower than this, there will be an increased margin over that calculated.

[ROW_BOT]

Is this for real? I’d expect a ring if one day you have to offload half of the aircraft in 40kt headwind, because you were “conservative” and took 15kt tailwind and 30C higher OAT, but not for the use of TOGA.
Yes unfortunately it was all too real in that company. FO's would beg you not to use TOGA, because they too had received a call to explain why they 'allowed' a Captain to do it.
Certain airline Managers see money saving as being more important than safety considerations, and have such little respect for their pilots that they question and second guess even the routine decisions they make if they 'cost money'.


This is a setting configurable by the EFB manager. He has the option in the setup to select 1. Show all results and highlight the flex or 2. Highlight TOGA and flex only.

The FlySmart software apparently has an option to display a range of several available Flex Temps as well as Max Flex and TOGA. In the company I'm referring to the EFB Manager (the one who rings up pilots for using TOGA) told me that he had disabled that option because he 'didn't want to confuse pilots'.
I kid you not!
The truth was it was to force them to always use Max Flex whatever the conditions. TOGA was out of bounds.

[FullWings]

There is conservative and then there is behaviour limiting on anxiety. As stated above, headwinds/tailwinds are factored in the RTOW tables or performance software.
I agree. If you ever use ATM/fixed derates, runway intersections, shorter runways when longer are available, etc. then your aeroplane is not performing as well as it could, in terms of absolute safety margins. But the ones you have are still adequate and acceptable.

The figures are such that if you go OEI at V1 with a wind component significantly worse than you used for performance calculations, it will still all work out. Most of the time the aircraft will perform better because of a lower actually density altitude than the one the sums used and gross/net differences. If conditions have changed radically since you ran the figures, airmanship would dictate you ran them again, if only for peace of mind?

I have no problem with “conservative” if there is a good argument as to why today is different. It’s like fuel: carry what you think you need but don’t just add it without thinking. Engine maintenance costs are extremely significant in modern operations and using more thrust “because I can”, not because it is required/sensible, is unprofessional.

If you feel the urge to pad out the numbers, where do you stop? When enough fudge factors have gone into a calculation, the result can diverge from what is actually needed by a silly amount...

[Airmann]

The issue that I had/have with this conservative mind set is more that could it inadvertantly get you into trouble? (Otherwise I have come to accept that some guys are actually way to anxious in the flight deck)

Specifically, in the case I am referring to I noticed a 10-15 kt drop in takeoff speeds after going 'conservative'. The issue is what are the implications in the event of an engine failure? The slower ground speed? Lower V2?

[OK4Wire]

One problem with using zero wind (if say the headwind is 5kt, and you want to go conservative) is that you can't get 50% of zero.

If, as you line up, you notice the wind is now 2kt tail, in theory you should either recalculate or go TOGA.

Especially where I tend to take off, where there seems to be a tailwind at both ends of the runway, I would probably have put in 4kt tail (which would allow up to 6kt actual). Going conservative by about 10kt seems reasonable to me.

[Derfred]

You mean You fly to controlled airports where You get a take off / landing clearance without any wind direction/intensity ?

Wind given with landing clearance, but not with takeoff clearance, which is the topic.

[Dualcouple]

One problem with using zero wind (if say the headwind is 5kt, and you want to go conservative) is that you can't get 50% of zero.

If, as you line up, you notice the wind is now 2kt tail, in theory you should either recalculate or go TOGA.


Exactly. People usually pay attention when close to perf limited TOW but hot/humid/high minimum flex takeoffs with headwind turning to tailwind can surprise too as the additional performance margin due to smaller TAS will be not be very large.

These are rarely spectacular on twins, but saw some interesting ones when flying A340-300 in aforementioned conditions. Personally I went for TOGA if not close to Vr within alternating white/red centreline light zone. No-one ever objected.

[Escape Path]

You guys did read when JT wrote that it’s half the headwind and all of the tailwind, not twice, what’s accounted for on the calculations, right?

I giggle a bit when I see these extreme “conservatism” for no good reason. Some even do it as a regular basis...
Perf margins are per se playing to our favor. Moreover, as some have pointed out, the performance margin is actually larger than calculated as the airplane (engines and wing) is operating in higher density than what the EFB spit out. I’ve seen some people reduce flex by one or two degrees but leaving the speeds for the original flex (since they shouldn’t differ too much) for a slight increase in margin.

I always recheck my numbers before takeoff to see if what I did on the gate is still valid. Airplane has OAT readings and an altimeter to check for temp and press and I should be on tower freq a couple of mins before takeoff to check for wind. They would give me wind info with my takeoff clarance so even at the latest, I could still check if my previous wind is still valid. Mentally, it doesn’t take longer than 5 secs to check this. Re run numbers if too different from initial calculations, just for peace of mind. I’ve already catched a couple of gotchas by doing this...

[john_tullamarine]

it’s half the headwind and all of the tailwind, not twice, what’s accounted for on the calculations, right?

Minor observation .. one and a half times the tailwind ...

[Check Airman]

I flew with a guy who'd ALWAYS use full power for takeoff (regardless of runway length, or other factors) if more than 80% of the seats were occupied. Sometimes, you just can't argue with stupid.

[Escape Path]

it’s half the headwind and all of the tailwind, not twice, what’s accounted for on the calculations, right?

Minor observation .. one and a half times the tailwind ...

Noted. Thanks JT. Dropped the ball on that one

[john_tullamarine]

.. as we all do from time to time ...