What happens if you take off with a direct tailwind?

Question

What happens if you take off with a direct tailwind?

Answer 1

Given how few bounding criteria you have in your question, the general result is

• For fixed wing aircraft: your ground speed will be significantly higher than if you'd taken off heading into the same wind, and thus you'll tend to use up more runway before lifting off.

  Example: with a take off speed of 100 kts, going into or downwind is the difference between 90 knots or 110 knots ground speed before reaching take off speed when the wind down the runway is 10 knots. This difference becomes more pronounced as wind speed increases; it's a differential of 2 x wind speed.

  With a long enough runway it won't matter in terms of "taking off before running out of runway." On a short field it may matter a great deal.

• For Rotary Wing aircraft: you may need a bit more power to hover when lifting off with a tail wind. The FAA has this guidance:

  Headwinds are the most desirable as they contribute to the greatest increase in performance. {snip} When taking off into a headwind, effective trans lational lift is achieved earlier, resulting in more lift and a steeper climb angle. When taking off with a tailwind, more distance is required to accelerate through translational lift.

For a variety of aircraft types, if the tail wind is high enough your manual will recommend against/prohibit down wind takeoffs, but that varies enough between models and types that a general answer can't encompass all of the variations.

Answer 2

So I will focus on fixed wing aircraft, as that's all I really know about.

Lift is a factor of your "Indicated Airspeed". So the faster the wind is going over the wing, the more lift the wing can generate. You always need a certain Indicated airspeed to take off. The plane simply won't "go up" until you reach that speed. Depending on the aircraft, the needed speed of the air over the wings changes.

For example in a Cessna Grand Caravan you need about 80 knots of Indicated airspeed to really take off. There are some "rules" in its handbook about short take-offs, but in general you want to be above 80 knots IAS before you start "going up".

Now if the wind is constant, and blowing from the front of the craft to the back at 5 knots, then you already have an IAS of 5 knots. Sitting still your IAS is 5 knots, because the air is passing over your wing at 5 knots.

If that wind is passing from the back (tail) or the aircraft to the front, then you have a IAS of -5 knots when sitting still.

Now we get into ground speed. Mostly when you're flying (in the air) you don't really care about ground speed, other then efficiency and as a measurement of where you are. If you care about it at all it's way back at the end of the list. You care more about IAS (to keep you up) and True Air speed (the speed of which you're actually moving through the air). But when you're taking off or landing you care quite a bit about ground speed.

So generally 1 knot of IAS would be equal to 1 knot of ground speed. So to get our sample plane off the ground, on a day with no wind, you have to be moving at 80 knots on the ground. That's roughly 92 miles an hour.

With a headwind (from front of the plane to back) of 5 knots you have to be moving at 75 knots ground speed or 86 mph.

With a tailwind (back of plane to front) of 5 knots you have to be moving at 85 knots of ground speed or 97 miles per hour.

Depend on your how your plane is designed, those few knots of ground speed can mean the difference between a good take off, and running out of runway. Or because the wheels are actually on the ground, exceeding the safety of the structure of the landing gear.

Lets take a runway of 2000 feet. There are smaller runways, though to be fair this is a pretty small one. The example plane needs 1,160 feet to take off in a situation with no wind and empty. So we could take off from this airport. There's a "rule of thumb" out there that says "operation with tailwinds up to 10 knots, increase distances by 10% for each two knots [of wind]." So at 5 knots of tailwind I go from needing 1,160 feet to about 1,500 (error on the side of caution). I can still take off, but what about weight? Different weights need different lengths to get up to speed. Another rule of thumb "10% change in weight will cause at least a 20% change in takeoff and landing distances." So for are sample aircraft lets say I have a total weight of 6,000 lbs. The Empty weight is 4,550. That's a weight change of around 31%. So now I need to add on 60% distance to my ground roll. So I need 1,900 feet to take off (again pad for caution). I'm still good. But let's add in the wind. Now I need 2,375 feet to take off and the runway just isn't that long. So I'm stuck on the ground.

So our example in summary:

• With no wind. I can take off
• With a headwind, I can take off
• But with 5 knots of Tailwind I can not take off.

SUPER IMPORTANT NOTES

• Rules of thumb are great, but they're not the POH. Every airplane should come with a POH (and I mean every physical plane should have a POH in it). In that POH are real limits, and real rules/guidelines. Most will contain a table or chart that spells out weight, ground roll and wind speeds in one way or another. Some craft will have safety limits, that should not be exceeded.
• The rules of thumb are great for conversations in general but should not be applied to an aircraft in real life. If you're flying a plane you should know the plane's specs and limits. Not just "guess" at them.
• The numbers I used in the example are accurate enough for an example, but come from Google.
• Runway length is also effected by what it's made of. Grass fields take off "longer" then asphalt.
• Runways can be as short as 800 feet, but most larger airports have runways in the 10,000 feet range. The longest public runway is about 18,000 feet. There are runways that are longer.
• While the runways may be bigger the jets that use them need to go faster and take longer to get there, so the problem exists for them, too. More so, in some ways.
• A lot more then just IAS goes into a take off there is air density, for one. The slope of the runway, the temperature of the air. This example is really simplified to look at the one aspect.

Answer 3

Two risk factors which have not yet been mentioned: The higher groundspeed required before airspeed is sufficient for take-off will put higher strain on the landing gear and wheels. Imagine a bump in the runway; hitting it at 80 or 100 mph will make a difference which in the worst case may cause e.g. a wheel puncture.

Also, should you need to abort the take-off, the braking distance before the airplane comes to a stop will be much longer from 90 mph than from 70 mph. The stopping distance will be 1.65x longer, although the speed difference in this case is only 1.28x.

Came to think of a third risk factor too: Wind gradient. Often the wind blows stronger the higher you get above the runway. This can be significant at small airfields. In a normal take-off with a headwind this helps the airplane climb better as it ascends. But in a tailwind take-off the tailwind increases as the airplane ascends, meaning that the climbing rate is reduced (nose must be held down to achieve sufficient airspeed for climbing). If the airplane has sufficient power this may not be noticeable, but a marginally powered plane may have difficulties if there are obstacles close to the field.

Answer 4

You will need more runway. Once airborne, there will be no difference, apart from an unfavorable wind gradient. Besides, it is not advisable, because, in non-controlled airfields, the pilots consider the runway with a headwind as the active runway.

Answer 5

The airplane will still fly; it just takes considerably more runway to depart. Most Pilot's Operating Handbooks (POHs) will use a phraseology in their takeoff roll performance charts such as "increase ground roll 10% for every 2 kts of tailwind" or similar. Doesn't look too bad but consider now that if you depart in a direct 10 kt tailwind, your ground roll and distance to clear a 50 ft obstacle is now 50% greater than a takeoff in still air. This become a serious factor on short airstrips and high field elevations and density altitudes. A Cessna 172 may require 1300 feet of runway to do this at sea level at STP but might be upwards of 5000 feet of runway needed for a 6000 ft field elevation at 30° C.

Answer 6

Jump-pilots and Ag-pilots regularly take off downwind and/or with strong cross-winds providing of course the runway is long enough. A dedicated airport is preferred. If you want to get tremendous experience as a pilot, fly 500 hours taking skydivers up to 11,000 feet. You are dropping falling bombs that become floating bombs that pass through winds aloft from different directions at different altitudes then land on the drop-zone target. You'll know you're a respected pilot when the experienced jumpers fall asleep during the climb. You will learn how to say NO! when your boss pushes you to the limit with weight, weather, and flying with minimum fuel. Most jump-planes look fairly junky and it's fun to tell first time jumpers when they see the airplane, "if you think the airplane looks bad you should see the parachute!"

Answer 7

"Once you lift off, there's nothing to worry about".... The increased ground speed may cause you to believe (optical illusion) you have a faster airspeed and the desire to increase your pitch attitude.(Which may be already high due to the lengthened take off roll and the need to climb). Your climb rate will suffer also. (vertical gain vs. horizontal travel). This goes against the bottom line...." Never regret NOT having the air above or the Runway behind you." My personal minimums will not accept a downwind take off OR landing ( also a major optical illusion) greater than 5 Knots.

Answer 8

The other answers and comments are ignoring an effect that usually does not show up in practice, but that it exists and I have witnessed first hand: as long as the air flow on the frame is reversed, so is the effect of the commands.

For take off, the effect diminishes as the aircraft gains speed, so it will probably only affect gliders and other light aircraft that rely heavily on the commands since the start of the run. On landing, the effect increases as the ground speed diminishes, and I have witnessed a serious crash (straight against a fence that run at the side of the runway) because of it.