TO performance 737NG wind/temp change ?

[president]

Hi all, this question is from a legal perspective, so please avoid "but that's airmanship comments". If you do your take-off performance with say 10 kts headwind on the EFB, are we allowed to depart if the wind drops to 6 kts head when we reach the holding point. I know it's taking into account just 50 pct of the headwind but can we "eat" that margin before we even press TOGA, or is a new calc required ? Same regarding temperature. We did the performance with for example 22 deg C. At the time we get to the holding point temperature is now 23 deg C. Are we legally allowed to depart without a new calculation. And QNH changes? What are the legal limits to when to perform a new TO call? If there are no limits, it doesn't make sense to always do the performance with actual condition from the ATIS (like it my current airline). Would be wiser to add a bit of margin to be able to accept small changes without the need for another calculation. I'm asking what's legally correct here, any not what common sense tells you. Looking for documentation preferably. Hoping for some positive inputs :)

[BleedingAir]

We have very specific margins within which we're not required to recalculate. From the data used for calculation, we're allowed a 4 kt decrease in headwind, no increase in tailwind, 1 deg increase in temp, 1 hPa drop in QNH. Anything outside these and it's back to the EFB.

[B737900er]

President - most practical on the line stuff is never written down because no one wants to be held accountable when it goes pear shaped. Boeing writes "advisory" for this very reason.

It is always down to the commander of the aircraft to be satisfied he has complied with the legal minimum performance for his aircraft, that will be the answer you will find in any legal document.

Now, if you was tight on the performance by a 100-300kg then checking the performance would be a good idea. If you have 2000kg then a small change wouldnt change the RTOM by much.

[RAT 5]

President: I understand your question; legal yes/no. I do not know the margin of error allowed: i.e. the margins of allowance with changes. However, the debate has widened to ways that can allow changes to happen without recalculation and avoid the 'can we/can't we' conference.
One wonders if the use of i-pads has dulled peoples thinking. With tables it was common, if no critical performance consideration, to use zero headwind for assumed temp thrust calculation. It would be a bonus. There would be some captains who would reduce to a couple of degrees before the absolute limit. You now have a couple of buffers to allow changes. If the tailwind was 5kts you'd use 10kts. If there was an intersection possible you'd use that performance even if you were expecting full length, just incase ATC offers it. Being proactive, anticipating, covering last minute changes to avoid delays. Airmanship. I wonder if the use of computers has led guys to calculate to the last digit when it's not necessary without thinking?

[president]

Thanks RAT5 and others. I personally always use a buffer to allow for changes. Like I said, this was just from a legal perspective. Can we depart with less headwind than calculated, legally? Most guys I see use the exact figures on the atis to calculate, based on perhaps 30 mins old values. You could have temperature changes of 4-5 degrees and much different wind components. I think it is wise for any company to have guidelines for, when we need to do a re-calc. Makes it easier for everyone with a set of rules, except just the opinion of the individual in the left seat.

[RAT 5]

Makes it easier for everyone with a set of rules, except just the opinion of the individual in the left seat.

Not sure that is always the best solution. There are the more enlightened airlines who trust their crews and would prefer the LHS chappie on the spot to use experience, judgement and discretion to do what is best for the situation. It would be sad day when the whole operation is conducted in a straight-jacket of rigid rules.

[president]

Agree, hardly anything is ALWAYS the best solution. But in this highly controlled environment it surprises me, we are not controlled or even guided in this case. It could save you the day you have forgotten your brain at home and things are marginal. Same reason why we always use checklists, although we know them by heart. My experience is that 90 pct of crews just punch in the figures blindly. A set of rules is in my opinion a good thing to have, as long as you know you can deviate from them, if it's in the interest of safety. We can and sometimes must deviate, so I don't see the problem. Nothing bothers me more than cowboys in the cockpit.



Makes it easier for everyone with a set of rules, except just the opinion of the individual in the left seat.

Not sure that is always the best solution. There are the more enlightened airlines who trust their crews and would prefer the LHS chappie on the spot to use experience, judgement and discretion to do what is best for the situation. It would be sad day when the whole operation is conducted in a straight-jacket of rigid rules.

[BleedingAir]

Well I've given you the figures outside which our crews must "legally" recalculate performance (B738), they're hard numbers and I think they're important, while there's no replacing basic judgement calls from the LHS, it removes the option of "yeah the temp's gone up 7 degrees, but it's a 4km runway, we'll be totally fine", which in my opinion is the first hole in the Swiss cheese, and every Captain has their own "comfortable limits", some a lot more "comfortable" than others. Not sure if these are operator or Boeing figures.

We never blindly punch in the ATIS figures without consideration. For example, if the temp is 10 degrees but it's 9am, we're not departing for 30m and the sun's blaring, we'll use, say, 14 degrees, which gives us up to 15 before a recalc is required.

[Goldenrivett]

president - You could have temperature changes of 4-5 degrees and much different wind components.
If you calculate your performance using FLEX, provided the actual OAT is less than the value you assumed, then you will still have more than adequate performance.

If the wind is much different - then re-calculate.

[scifi]

If you have got the first set of figures from the (20 minutes old) ATIS, where are you going to get the current figures?


Surely its a bit too late when Tower gives you.. 'Clear to take-off, wind 120 / 12 knots'

[president]

@ Goldenrivett,

I am not using FLEX but if it´s the same as assumed temperatures on a Boeing, I think you are dead wrong.

We are departing light weight in -10 deg C with assumed temperature 65 deg. You are telling me that the temperature can increase 75 deg without having to re-calculate?

The whole concept of assuming a temperature comes from the fact that the engine is flat rated to typically isa +15 deg (around 30 deg C OAT at sl.). Up to that temperature the engine will produce the thrust rating on the engine, and not more. It's limited by internal pressure, engine temperature limits and fan speed limits. After ISA +15 the thrust drops linearly with temperature. You are now telling the engine that what the actual temperature is, and what temperature you want it to assume it is. If you assume the temperature is OAT 45 deg C, the thrust calculated will be equivalent of the full rated TO thrust at 45 deg, but at a much higher N1 setting. In other words, to produce the same thrust (that you want at 0 deg) at 45 deg, the rotor would have to spin quicker. We are on the declining line on the flat rated engine thrust versus temperature now, so we are not N1 rotor limited here, but engine temperature limited.

Imagine you calculated a take off at 0 deg OAT and an assumed temp of 45 deg, your N1 value would be way off if the actual temperature increased to an actual 45 deg C. I´ve done the calculation for you on the EFB right now. On a given take off at 30 deg C it came out with 22K assumed 38 deg and an N1 of 91,3 %. If the temperature goes up to actual 38 deg C, it gives me full 22K thrust but an N1 of 92,5. Say that you took off at 38 deg with 91,3 (like I think you suggest), you would end of with a too low thrust setting. Also from this is given that 91,3 % N1 at 30 deg OAT gives you the same thrust at 92,5 pct N1 gives you at 38 deg OAT.

We do though have what´s called the call the "true airspeed effect" giving you better performance that calculated. We tell the engines that we have a temperature of 45 deg C (thin air) air, but the actual temperature is 0 deg C (thick air). But this does NOT mean you can accept temperature increases up to your assumed temperature limit.

[president]

@scifi,

Quite often conditions change on the way out. QNH changes from ATC, wind sock, previous aircraft getting their TO clearance with the wind. I am not talking about the cases where you have no reason to think that things have changed. How many times have you taxied out and runway in use changed? Because conditions change. If you got a runway change, would you just change the runway on the EFB and recalculate without taking into account that the wind changed 180 deg? It´s exactly what I am talking about here, just without the rwy change.

thanks everyone for comments, seems like there is no decisive legal stuff, and that it is up to the operator to pose limits on this.

[Goldenrivett]

@ president
I think you are dead wrong.
Say that you took off at 38 deg with 91,3 (like I think you suggest), you would end of with a too low thrust setting.

I think you are confused.

Our FADECs sense the actual OAT and adjust the target N1 accordingly. There is no need to change the assumed temperature.

If your FADECs require you to enter the actual OAT in order to match the reduced thrust you'd get at the assumed temperature - then do it.
The OAT may change, but your calculated assumed temperature limit won't.

[president]

I did not know that the bus updates the temp and hence N1. We don't have that feature on the 737, so we would end up with wrong N1 setting if we touch nothing and temp goes up. Of course if you chg the OAT in the box we can accept up to our assumed temp. But we have to do that manually and thereby delete our TO perf.

@ president


I think you are confused.

Our FADECs sense the actual OAT and adjust the target N1 accordingly. There is no need to change the assumed temperature.

If your FADECs require you to enter the actual OAT in order to match the reduced thrust you'd get at the assumed temperature - then do it.
The OAT may change, but your calculated assumed temperature limit won't.