Headwind and Tailwind in Takeoff/Landing Performance

[Bubi352]

I need a clarification. I read the CAA safety leaflet 07 pertaining to airplane performance for general aviation. It says if there is an adjustment that can be made for wind, only 50% of the reported headwind and 150% of the tailwind should be considered to account for variables. Now, what confuses me and I was told this before, is that most performance graphs account for this "rule". I have looked through several AFM for different manufacturer and I have not found any reference to it. My question is:

1) Do performance graphs/tables, include those factors? Which manufacturer have included them?

OR

2) Should we simply account for no more than 50% of the reported headwind and 150% of the tailwind and plug this in the graphs or tables to find the takeoff or landing performance?

[john_tullamarine]

The 50/150 wind component requirement appears in the design standards. As such it should be built into the AFM charts.

How do you tell ?

Have a look at the chart (if the AFM present data in tabular format you may have to draw a quick sketch to see the effect described below) -

Subject to how the OEM has put the equations together,

(a) if there is a discontinuity (ie the slope of a line changes noticeably) at zero wind, then the factors are included. This may be a little difficult to see at the lower weight/distance end of the wind carpet but will be quite obvious at the other end.

(b) if there is no discontinuity and the wind line is nice and smooth across zero, then it is not.

[Old Smokey]

John_T speaks the truth, as usual, the 50%/150% factors are built in to the Takeoff performance data given you, there's no need for you to apply any correction to the actual wind. A point or two to remember however......

(1) The 50%/150% factors give you a bit of "fat" for the Takeoff calculation. In calm wind conditions, you have no built-in "fat" at all. (50% or 150% of Zero is still zero).

(2) Pre-flight number crunching is typically done 20 to 30 minutes prior to Takeoff using the ATIS wind or your own observation if ATIS is unavailable. If, at the time of actual Takeoff, the headwind has decreased, or the tailwind increased, then, if the change is less than the built in margin (e.g. 8 Kt Headwind instead of 10 Kt), then aim it and go, you are merely using a little of the built-in margin. (ATC typically provide a wind check included with the Takeoff clearance). If the wind component has changed by MORE than the built-in margin, it's time to do a recalculation before departure. (The latter is most appropriate if you were at performance limiting weight or you are using Reduced Thrust (Assumed Temp / Flex). It amazes me how many people do not do this last moment check:confused:

Regards,

Old Smokey

[mutt]

Do you actually use the headwind in your calculation?

Mutt

[tom35]

Regarding the take off performance, there 's no question: Use the head wind.
For landind it's a little bit different, and it's an airline policy, actually you have the choice between flying Vref over threshold and use the head wind correction, or flying Vapp. In that case, overspeed and head wind will counteract so you can't recalculate your landind distance with headwind, you consider no wind performance. Embraer works like that, in the AFM they write there's no difference between a landing at Vreff without wind, and a landing at Vref+20 with 20 knots of head wind. Of course you can fly your approach at Vref and recalculate your landing perf with 20 knots of head wind, but your margin regarding stall is not comfortable especially without auto-thrust.

rgds

[Old Smokey]

Hi Mutt,

Yup, we do now but didn't before.

In the 'paper' Airport Analysis era, we always assumed a Headwind to be zero, and used the Zero wind data from the RTOWs. Of course, Tailwind was ALWAYS considered.

Now, in the EFB 'Performance Tool' era (hate that name), policy is to use the exact data to fill all of the boxes. Although only in service for about a year, we're on about Mk VIII instruction how to use it:ugh:

For my own operation, I used to put in the LEAST favourable wind, e.g. if the wind on RWY 02 was 020/10 varying between 350 and 060 degrees, I'd insert 060/10, the least favourable component provided by ATIS. I was reminded on a line check that this was non standard, and should use the basic wind. I nodded wisely in agreement, and reverted to my 'least favourable' concept between line checks.

When the new toy becomes old, common sense will again prevail.:ok:

CAVEAT - Sometimes you need to use the Headwind to operate a 'heavy' sector, 99% of the time you don't.

Regards,

Old Smokey

[icefire1142]

can anybody comment the rule of thumb re tailwind takeoffs in most ga aircrafts saying 5% increase for every 2 knots tailwind? been operating with this condition for nearly a year now.

[Pilot Positive]

Bubi 352 What performance data archive does your operator use?

Not sure if it helps or not but Flygprestanda Flygprestanda AB - Performance Engineering since 1969 (http://www.flygp.se) can really help you with performance for most airports. It factors all MAT limitations + HW/TW components as well as commercial considerations for dry/wet runways for your aircraft - all at the touch of button (Phew!)

I'm sure there other products out there - our operation uses this one as standard.


:ok: