# How do airplanes measure cross-wind?

Are aircraft equipped with any instruments that measure actual crosswind velocity during a flight? What is that instrument called and is it used by autopilots to apply wind correction?

• Worth noting that when aircraft are vectored by ATC they're given a heading which includes any required corrections. If ATC says Heading 220 they mean your nose should be pointed at 220 - you aren't meant to second guess and add any corrections. I'd wager most use of autopilot is by aircraft operating under such conditions so I doubt any apply any crosswind corrections
– Dan
Nov 21, 2021 at 22:55
• @Dan If you tell the autopilot to fly a given heading, then it flies the given heading (of course without any correction applied). But some autopilots also allow flying a given track instead of a heading (e.g. on an Airbus FCU you get that by pressing the HDG-TRK V/S-FPA button), which then has the correction applied. Nov 22, 2021 at 9:55
• What sort of aircraft, and under what conditions? For most GA planes flying VFR, it's called "looking out the window". (Especially when landing.) Nov 22, 2021 at 17:35

There is not a specific instrument for this, because it is not directly measurable.

Most modern aircraft have digital sensors for heading and airspeed, and their GPS/IRS will know ground track and ground speed. Any difference between those two vectors will be due to wind, and it’s a simple calculation to determine the wind vector’s direction and magnitude.

In practice, pilots (or autopilots) know the desired track, so we simply adjust where the nose is pointed until the actual track matches the desired one. There’s no need to know the exact winds.

I’ve flown planes with a wind display, and the only thing I really used it for was when practicing engine failures, to maximize glide distance with a tailwind and then minimize landing distance with a headwind. Otherwise, it was for entertainment value only.

Yes, it is called a driftmeter and consists of a telescope aimed straight down at the ground. It has a set of scribed lines projected onto its field of view which the navigator can rotate so the scribes exactly follow the paths of objects on the ground. The navigator then reads off the resulting angle between the plane's axis of motion and the scribe lines. Knowing the plane's airspeed, the navigator then solves the resulting wind triangle to obtain the crosswind component.

This is a simplified explanation; in practice the navigator also needs to know whether the plane is in a tailwind or headwind to solve for the exact value of the crosswind.

• Your last sentence is weird. Driftmeters were used to improve dead reckoning in areas where radio navigation isn't available (e.g. over oceans), so radio navigation hasn't eliminated the driftmeter. I think you mean inertial navigation and GNSS have eliminated it. Nov 22, 2021 at 9:27
• Today drift is measured by the ADIRU comparing air probes data and accelerometers data.
– mins
Nov 22, 2021 at 15:44

Not crosswind per-se but wind in general is calculated on-board by comparing aircraft internal data with GPS data. Your aircraft knows it's current heading and most computers can give you a pretty accurate educated guess on your True Air Speed. If you compare both of those with your Ground Speed and Track (this is, the actual lateral flight path of your aircraft) then you know your Drift and the speed difference. This is now simple math. "Aircraft" Vector + Wind Vector = Flight Path Vector so Flight Path Vector - Aircraft Vector = Wind Vector.

Modern airliners have Flight Management Computers that automatically calculate the crosswind component and then apply any necessary course corrections in any Lateral Navigation mode.

Many can also display the current crosswind component.

Here is an example of a B777 FMC that shows a right crosswind component of 28 knots in the upper right corner.

Airplanes with FMS systems normally display calculated wind direction and speed on the navigation display. It's simply based on the differences in heading vs track, and true airspeed vs groundspeed, worked out within the FMS computer.

You can watch it in action in real time if you fly across a jetstream on a perpendicular track. You will see the heading change several degrees upwind but with little change in groundspeed because the wind is at 90 degrees, but the FMS will calculate the velocity of the jet from TAS and the heading vs track change, and you'll see the wind velocity rise from, say 30kt, to 90 kt or more, with the airplane turning into it to maintain track as you enter the core of the jet, then back to what it was originally when you exit the other side.