Using 'Conservative' Performance Numbers

[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]

Originally Posted by sonicbum

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]

Originally Posted by 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.

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]

Originally Posted by 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 ...

Noted. Thanks JT. Dropped the ball on that one

[john_tullamarine]

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