Headwind & Takeoff

[dream747]

I came across a question which answer states that a headwind increases the clearance of obstacles but it does not affect the mass, altitude and temperature limit for departure. I understand that headwind gives an increase in climb gradient which gives the improved climb performance but why can't I increase the loads I can take for departure since a headwind decreases the takeoff roll for a given weight, and given the fact that I have extra height available for obstacle clearance?Sounds simple but I can't get around it!Thanks for any inputs!

[barit1]

If the surface wind velocity remained constant up through obstacle clearance height, then it would be logical to take credit for the headwind.

But I believe the perf. manual assumes that the headwind is not reliable above the surface - ergo no credit.

But it's been a while...:uhoh:

[SNS3Guppy]

If you could count on the headwind not varying or quitting, then it might be reasonable to take some credit into account when calculating performance...but what are you going to do when you've increased your takeoff weight for the wind, and the wind dies?

A headwind, like reverse thrust, is an extra gift, never to be depended upon. If it's there, dandy. If it's not, you're out nothing because you haven't relied upon it.

[kijangnim]

Greetings,

In any case the takeoff perf takes only 50% of the headwind entered in the graph, whereas it increases the tailwind by 100%.
Wind it known to be changing unevenly during climb. :ok:

[Fullblast]

whereas it increases the tailwind by 100%.


You wanna say that in case of tailwind you have to take it in account at 100%?
If it is, wrong, you have to take in account 150% of tailwind.

FB

[kijangnim]

Greetings

The Windspeed head and tail corrections (minoration and majoration) are embeded in the take off charts, you just enter the wind as reported by tower ATIS VOLMET ...to the graph and read.

[eckhard]

Hi dream747,

I believe you are 'mixing apples with oranges' here. There are several performance criteria to comply with for take-off but they can be broadly split into two groups:

1. Runway performance; and
2. Climb performance.
(3. Structural or Certified Weight limits must always be complied with, natch.)

Under 'Runway performance' you would include things like Field Length limits, brake energy limits, tyre speed limits and, yes, Obstacle limits. For all these you take into account runway length, wet/dry, slope, QNH, wind (50% or 150%), Altitude (Field Elevation) and Temperature.

'Climb Performance' has nothing to do with runways or obstacles. It is simply a regulatory requirement to achieve a minimum gradient of climb in various configurations and at various engine settings (and numbers of engines) for different climb segments. These requirements are also referred to as 'WAT limits', as you need to take into account the Altitude and Temperature (and QNH) when establishing the Weight. No mention of winds.

The reason for the WAT limits is this:

Imagine you are taking off from a perfectly flat, dry sea-level airfield with no obstacles for 100 miles (over the sea perhaps). The runway is 15,000 feet long. So for most aircraft there would be absolutely no 'runway performance' issues. However, the regulations still insist that you have to be able to climb away after an engine failure at a minimum gradient, so that you don't rely on the curvature of the earth to gain height. For example, on a twin-jet, you must be able to achieve 2.4% at V2 with the gear up, flaps at take-off setting and one engine inop. There are other gradients listed and they differ for three or four-engined aircraft, but you get the idea.

As these are pure performance requirements and are not related to the geography of where you are, there is no need (or credit given) to take winds into account.

Hope this helps,

Eckhard:8

[barit1]

I won't dispute that a bona fide headwind INCREASES the effective climb gradient - If you're at the performance limit of 2.4%, a 10 kt headwind might boost that to 2.5% (because GS is lower, it's taking longer to reach that obstacle, yet your ROC is unchanged.)

The whole question is - is that headwind dependable? From a cert performance standpoint, the answer is always NO.

[SNS3Guppy]

In any case the takeoff perf takes only 50% of the headwind entered in the graph, whereas it increases the tailwind by 100%.


That would really depend on the aircraft and performance system in use.

The Windspeed head and tail corrections (minoration and majoration) are embeded in the take off charts, you just enter the wind as reported by tower ATIS VOLMET ...to the graph and read.


Again, that would really depend on the aircraft and performance system in use.

[john_tullamarine]

Some innovative answers in this thread ... and some which are right on the mark .. in summary

(a) WAT climb limits are "line-in-the-sand" minimum capability limits established for nil wind conditions .. see, for instance, FAR 25.101 (http://ecfr.gpoaccess.gov/cgi/t/text/text-idx?c=ecfr&sid=bb984cf119879f6f25312bb3848f1dba&rgn=div8&view=text&node=14:1.0.1.3.11.2.155.8&idno=14) and FAR 25.121 (http://ecfr.gpoaccess.gov/cgi/t/text/text-idx?c=ecfr&sid=bb984cf119879f6f25312bb3848f1dba&rgn=div8&view=text&node=14:1.0.1.3.11.2.155.18&idno=14)

(b) for other calculations, the normal standards are to allow 50% for headwind and require 150% for tailwind .. see, for instance, FAR 25.105 (http://ecfr.gpoaccess.gov/cgi/t/text/text-idx?c=ecfr&sid=bb984cf119879f6f25312bb3848f1dba&rgn=div8&view=text&node=14:1.0.1.3.11.2.155.10&idno=14) for TOD calculations

(c) in respect of the wind factors, the AFM charts normally have the data built into the chart. This can be determined, quite easily, by inspection .. the effect of the changing factor is to cause a significant discontinuity (slope change in the lines) at zero wind

[dream747]

If the climb gradient demands a certain % due to obstacles, logically there must be a decrease in the take-off mass to acheive this climb performance. This I would suppose give a surplus TORA and TODA. Since V2 is the take-off safety speed which I assume varies with weight, and in this case will decrease because of the decreased weight.

Is there a procedure then to increase V2 to the speed near the best climb angle speed without offloading anything?

I don't know if it sounds confusing sorry if it does but thanks everyone for all the inputs. Really appreciate it!

[john_tullamarine]

there must be a decrease in the take-off mass to acheive this climb performance

..will depend on the circumstances but often (usually) is the case

This I would suppose give a surplus TORA and TODA.

need some care with terminology .. TORA/TODA relate to what the runway has to offer and are aircraft-independent. If you reduce weight to make a gradient requirement (at min speed schedule), then all the required runway numbers will reduce .. ie TORR, ASDR, TODR

Since V2 is the take-off safety speed which I assume varies with weight, and in this case will decrease because of the decreased weight.

down to the point where speeds become Vmc limited

Is there a procedure then to increase V2 to the speed near the best climb angle speed without offloading anything?

not necessarily a case of pegging best climb angle but we can improve climb performance .. Boeing calls it "V2 overspeed" and Airbus "improved performance". Min V2 schedules are well below best rate .. it is not feasible to go to max rate as the benefits get eaten up by runway requirements but, in general, it is useful to increase up towards, say, V2 + 20 or 30 knots if the runway length permits.

I don't know if it sounds confusing

.. don't worry .. we're all a tad confused at times ..

[dream747]

John thanks for the clarification.

For the case of the minimum climb gradient set by the relevant aviation authorities, for example 2.4% for the 2nd segment climb for twin jets, I suppose the charts would give the necessary weight limitations given the TORA, TODA, current temperature and pressure... etc so the aircraft will meet the 2.4% gradient upon the failure of an engine.

What if the climb gradient required to clear obstacles is increased to 6% for example? If an engine fails and terrain ahead of you requires that 6% to clear I'm sure 2.4% is not a problem here and there's no room for lateral clearance, how would the pilots deal with such a situation?

Many thanks once again! Appreciate all the help:ok:

[SNS3Guppy]

The takeoff runway distance available really has no bearing on the percent climb gradient...that's a function of weight and power and temperature...but the takeoff distance required, and the ability of the airplane to meet a particular climb gradient with or without an engine failure are separate subjects.

If a higher climb gradient is given than the minimum, then one calculates one's weight to be able to meet the required climb gradient, or chooses a different departure procedure, or does whatever else may be necessary to operate safely (choose a different runway, chose a different time of departure to take advantage of lower temperatures, etc).

Waiting until an engine has failed is not the time to be concerned about whether the airplane will meet the climb criteria. That's part of the calculations performed prior to departure.

[Stuck_in_an_ATR]

I'm starting to get confused in this as well, but my understanding is that:

- you USE the headwind (or 50% of it) for RUNWAY and OBSTACLE limits
- you DON'T USE the headwind for climb gradient (WAT tables)

The RTOW is the minimum of the three limits above. So, if the required obstacle gradient is greater than the one from WAT (2.4% for twins), the headwind is accounted for and actually increases the RTOW

Correct?

[eckhard]

Yes, that's basically correct. Having re-read my previous post I should clarify as follows.

To comply with the regulations, your RTOW is the lowest weight that can comply with all of the following:

1. Structural or Certified Maximum Take-off Weight.

2. Climb requirements or WAT limits (nothing to do with obstacles)

The tables in the AFM will list limiting weights at various combinations of altitude and temperature. Note that there is no mention of wind component because it is irrelevant in this case. To be precise, you should use the pressure altitude (i.e. correct for non-standard QNH) and temperature. If your planned TOW is equal or less than this listed weight, you will comply with all required WAT climb gradients (but not necessarily clear any obstacles).

3. Runway Length requirements

The AFM will give tables/graphs to show the maximum weight that can comply with the requirements to:
a) Accelerate and stop in the ASDA (runway + stopway)
b) Accelerate and go (35ft dry or 15ft wet) in the TODA (runway + clearway)
Both a) and b) assume recognising an engine failure by V1.
c) Get airborne with all engines by the end of the runway.

The AFM graphs/tables will take into account wind component, slope, pressure altitude, temperature, wet/dry surface. All these factors are also allowed for the following:

4. Brake energy limits;
5. Tyre speed limits.;
6. Vmcg limits; and

7. Obstacle clearance.

The AFM will have charts showing net climb gradients and/or total distances required for the 4 climb segments, starting from 'reference zero'. Reference Zero is 35 ft (dry) or 15 ft (wet) above the departure end of the runway.
It will also show the effect on these gradients of a steady turn of more than 15 degree heading change, in case you want to construct an 'escape path'.
Having this information enables you to construct the 'net flight path' to see if you will clear the relevant obstacles.
Once again you are allowed to take into account wind component, slope, pressure altitude, temperature, wet/dry surface (to decide whether ref zero starts at 35 or 15 ft).

If the operator can afford it, some clever computer programmer puts all the data in and comes up with the most limiting of 1 to 7 above. You are given one weight which is the most limiting of all the 7 and make sure you load the aircraft accordingly.

If money is tight, the pilot or dispatcher gets out the AFM and the local topo maps and spends about two hours working it all out. Or more realistically, the pilot simply checks the runway length against some sort of quick reference chart and goes.

So to summarise and get back to your original question:

"I came across a question which answer states that a headwind increases the clearance of obstacles but it does not affect the mass, altitude and temperature limit for departure."

Now I hope you can see that the gradients required for obstacle clearance and the 'minimum performance' required by the WAT limits are two different things; as I said earlier 'Apples and Oranges'!:ok:

[dream747]

I have it absolutely in clarity now. Simple once you've got it, get thrown with too many requirements and factors and I get it all confused. Thanks everybody.:)

[john_tullamarine]

the headwind is accounted for and actually increases the RTOW Correct?

Not quite ... the WAT limit takes precedence over the runway or obstacle limits.

Eckhard has the story reasonably well described ...

To comply with the regulations, your RTOW is the lowest weight that can comply with all of the (limiting considerations)

that's it in a nutshell ..

3. Runway Length requirements
c) Get airborne with all engines by the end of the runway.

This requirement is the TORR and the aircraft liftoff point is somewhat short of runway head. While there is some variation in the rules, typically, half the air distance to screen is over the TORA declared for the runway (normally the seal)

6. Vmcg limits

This applies to Vmca as well. Normally, appropriate use of the AFM/QRH will capture these limits. Generally, we don't get involved with them other than at very low weight takeoffs where the standard min speed schedule may become Vmcg/Vmca limited.

7. Obstacle clearance.
Reference Zero is .... above the departure end of the runway.

This would only be the case where the runway is limiting .. it is normal to run the analysis from the end of the TODR rather than runway head.

the effect on these gradients of a steady turn of more than 15 degree heading change

Not all AFMs provide turning gradient deltas. As a general guide, the delta will be around 0.5-0.7 percent gradient reduction.

gets out the AFM and the local topo maps and spends about two hours working it all out

or, for a complicated runway, spends two days or more doing the job manually ...

or, more realistically, the pilot simply checks the runway length against some sort of quick reference chart and goes.

suckerbait this suggestion ...


You may also be limited by enroute, approach, or landing limits .. in that these are migrated back to the RTOW considerations .. no point in departing at a weight too great to make the down river limitation ...