Tail and Head Wind Components

[likair]

Hello

Do usually aircraft takeoff uphill with a headwind component or downhill with a tail wind component?

Thanks for the info.

[Prop Job]

Hi likair,

It would all depend on what gives you the best overall combination. Ultimately you would be able to get that answer from the applicable aircraft's take-off graphs.

The following figures are from a C172 (sorry, it's the only figures I could get my hands on this morning), but the same principle would apply to other aircraft.

TAKE-OFF DISTANCE FACTORS
2% uphill slope - 10%
Tailwind component of 10% of lift-off speed - 20%

Unfortunately the C172 manual does not specify what advantages a headwind or downslope would give you.

If, for example, you want to take-off with a 2% uphill slope - a 10% increase in take-off distance - but you have a headwind that will give you a 15% decrease in take-off distance, it will obviously be more advantageous to take-off in that direction.

The same principle applies to downwind take-offs. If the decrease in take-off distance due to the downslope of the runway outweighs the increase due to the downwind take-off, it will give you an shorter take-off distance overall.

NB: It is important to remember if you choose to take-off downhill with a tailwind that your stopping distance will be greatly increased should you decide to abort the take-off!

All-in-all you will have to spend some time with the graphs to get the best option for the day.

Hope it helps,

Prop Job

[likair]

Thanks a lot for the post Prop job,

What I'm concerned of is, what is preferably chosen having the mentioned factors for take off.

I mean, if you have a 2% of down slope together with a parallel wind component with the slope, would you take off down the hill w/tailwind?

Takeoffs are preferably done down hill, increased speed while rolling the runway. On the other hand headwind is also important factor.

Thanks for your help.

[welliewanger]

Always try to take off with a head wind. It reduces the takeoff distance required. Aircraft can take off with tailwinds. As a rule of thumb, the faster the unstick (liftoff) speed, the greater the acceptable tailwind component. Up to about 10kts.

Before I go into slope limitations I need to define a couple of things:
TODR: TakeOff Distance Required. Distance from stationary to flying 50 feet above ground.
ASDR: Accelerate Stop Distance Required. Distance from stationary to just below V1 (maximum speed to abort takeoff e.g. if an engine fails). Then abort takeoff. Then stop the aircraft on the ground.

Slope limitations are +-2% (I think this is a certification thing. It's certainly not a physically limiting factor)

Up slope:
-TODR increases (harder to accelerate up hill)
-ASDR increases (harder to accelerate up hill, but easier to stop)
Down slope:
-TODR decreases (can accelerate faster down hill)
-ASDR increases (can accelerate to V1 faster, but deceleration is slower)

I've not done the calculations, but I'm pretty confident that the down slope option is preferrable.

Practically, it's not up to the pilots. ATC tell you which runway is in use, usually dependant upon the wind. Then you decide if you can go or not. If it's a no go, you do the calculations there and then since the results will vary depending upon how much head / tail wind and up / down slope there is.

[werbil]

The surrounding terrain is also a factor that needs to be considered.

I used to operate from a 1.5km strip with a 1.25% gradient. I very, very rarely took off up the hill for two reasons.

Taking off down the hill the ground continued to fall away, and once a couple of trees on the end of the runway fell over one day if you could clear the fence you had another 5km before you had to climb. At the other end of the runway, the gradient was around 5% for about 1km, and then got a lot steeper very quickly - there was a 1300 AGL ridge about 5km from the end of the runway.

Also, I used the experience severe downdrafts near the hills. I once calculated that if there was 15 knots of breeze blowing down the runway at the upwind end there was an AVERAGE sink rate of around 300fpm between the upwind end of the runway and the ridge.

Whilst ATC determine the preferred runway at controlled airport, if a PIC operationally requires a particular runway in Australia they will usually approve it without a loss of priority (providing you notify them early enough).

[Denti]

Often observed a Delta 767 doing a RWY 07 (downhill) departure in STR with a tailwind, it obviously was preferred over doing the opposite thing (upill with headwind). Actually we did the same if we were heavy there as the performance was still much better downhill with a tailwind than uphill with a headwind. It is actually one of very few runway situations where some airlines have an increased tailwind limit for take off (15 kts instead of 10kts).

In that case it is a combination of uphill slope and obstacle situation as there is a hill situated right behind the uphill end of the runway.

[SNS3Guppy]

There's more to the equation than the headwing/tailwind component, or even the slope of the runway. The obtacles or terrain that lies beyond the runway should be an important of the consideration, just as the type of aircraft involved. If you're talking about flat terrain around an airport with a slight gradient...then generally taking off into the wind is preferred. However, if it means an excessive taxi and the tailwind taking off the other way means a shorter taxi with less brake and wheel assembly heating, then the slight tailwind may be preferred.

If you're flying a heavily loaded supercub out of a canyon, downhill with the tailwind is preferred in nearly all cases; you go the way the water flows. It flows downhill, and so do you.

Suppose you have convective weather into which you'll be flying if you depart on the uphill, upwind runway? Going the other way may be your best bet...obstacles aren't always on the ground...it may be a cell, or another airport with high traffic density, or a departure procedure with a gradient that's unacceptable...again, many factors to consider.

Consider all the variables, and make your decision accordingly.

[likair]

thanks a lot for the replies!!

[Intruder]

For a transport airplane, you have to look into the performance book and runway charts or performance computer for the specific conditions. Either runway may be advantageous, depending on the degree of slope and magnitude of the tailwind component.

Also, if the tailwind is greater than the airplane's placard limit, regardless of slope, is isn't a legal option.

[Mark1234]

I have this stored in my bookmarks. No warranty as to it's validity(!)

Correct Choice for Shortest Uphill/Downhill Takeoff (http://pilotsweb.com/train/takeoff.htm)

[TyroPicard]

First it's Joe the Plumber, now it's Joe the Fisherman...