# How does an aircraft derive winds aloft on its own?

Aaron
• How does an aircraft derive winds aloft on its own? Aaron

The G1000 has a feature where it can put up a winds aloft arrow/speed in the PFD. I've been sort of wondering how exactly this is determined. My guess is that the aircraft (or the G1000) knows your airspeed via the pitot/static system and heading via the fluxgate/mag compass. Then, it uses your GPS position to figure out where you should be every X seconds given the airspeed and heading. Any difference must be attributable to winds aloft. Does this make sense?

• Yes, you are correct. You need three pieces of information:

• airspeed
• true compass heading (considering magnetic variation)
• multiple absolute position fixes (eg. from gps)

The aircraft heading has to be measured by a compass within the aircraft (it can't be derived from the gps track), and the gps position can be used to look up the local magnetic variation. The difference between the speed/heading and the ground track (in straight and level flight, anyway) is attributable to winds aloft.

• Close! Here is a simplified example:

• First we use GPS positions to determine the ground vector. This is based on distance traveled and ground track over time. It is a fairly straight-forward calculation based on the time to travel between two coordinates and their relative positions.
• Then we calculate our flight vector. This is based on the aircraft True Airspeed (TAS) and heading.1
• Lastly we use basic geometry/vector math to calculate the difference between the two vectors. This calculated vector is the wind direction and speed.

1 TAS is Calibrated Airspeed corrected for altitude and non-standard temperature - the speed of the aircraft relative to the airmass in which it is flying. It is calculated using the pitot-static inputs along with the Total Air Temperature (TAT) input (if available). To get an idea of the math, take a look at this answer of mine that calculates Mach Number, which is one of the values used to calculate TAS. Also, the TAT isn't really needed for low-speed aircraft that fly below about 100 KIAS.

Tags
• The G1000 has a feature where it can put up a winds aloft arrow/speed in the PFD. I've been sort of wondering how exactly this is determined. My guess is that the aircraft (or the G1000) knows your airspeed via the pitot/static system and heading via the fluxgate/mag compass. Then, it uses your GPS position to figure out where you should be every X seconds given the airspeed and heading. Any difference must be attributable to winds aloft. Does this make sense?

• In As the Pro Flies, John R. Hoyt writes (pages 41-42): Suppose we have to land in high, gusty winds. That's what happened to Pilot Z, who once landed his plane during such conditions with his flaps down. After the wheels were on the runway he relaxed, never realizing that a plane is not landed until the switches are cut. Because he still had airspeed and because full flaps lowered... on to state how much flap should be used in what conditions, and then he finishes with this: Let us then raise the flaps in gusty or crosswinds as soon as the wheels touch down. To wait until it is time

• There are various services that use world-wide Boeing Winds for forecast wind data which can be used to calculate an approximate flight time between two locations. They usually have best case, worst case, and average case for each location, altitude, and date in the future. I have searched and searched Google to no avail. Where can this wind data be found, and how can it be used in a commercial product? For those of you who don't know what the Boeing winds are, I found this description of their software product on am informal message board (not related to Boeing): PC WindTemp

• I know that under FAR Part 135, specific approval is required for an RNAV SID/STAR (given via an OpSpec), but what about Part 91? Does the aircraft/pilot have to be approved, and if so how is the approval obtained?

• The alpha vane is an external probe used to measure the angle of attack. I have been trying to understand how exactly it works, but I can't find any clear explanation or simulation. Is the vane static or dynamic i.e. does it rotate along its central axis? Given that it has a significant surface area, I think that it would either: Rotate because of the force/drag exerted by the airflow, and give an angle of attack proportional or equal to its angle of rotation Measure the force being exerted on it via a force sensor embedded in the surface Is either of these correct? In short, how

• for ATC to issue a descend via clearance, though I could be wrong about the second part. Some charts say "VERTICAL NAVIGATION PLANNING INFORMATION" before the expected altitude, so my assumption... though it's not required? How common is it for ATC to actually issue a "cross [fix] at [altitude]" where the fix and altitude are as expected on the chart? And, if it is common, why wouldn't it simply..., and does this show up in the FMS exactly the same as a mandatory crossing altitude? Are expected altitudes treated as suggestions by pilots or controllers? For example, in the above chart, if you

• 250 MPH and isn't generally covered in antennas, how significant is the antenna drag on a typical light GA airframe? For example, would stripping the ~6 pounds of antenna drag from your average...Antennas sticking out of an aircraft obviously increase profile drag, but the folks who design antennas have done a lot to improve aerodynamics: High performance aircraft can have antennas mounted inside of fiberglass components like wingtips, and flush-mounte antennas are available for transponders and DME equipment. For slower light GA aircraft more aerodynamic antennas are also available

• These are calculations which I use to know when to descend and the Rate: Multiply the ALT of feet to lose by 3 and $Groundspeed\div2\times10$ will give you your required rate of descent for a 3° glide slope. For example: FL350 to FL100 => 25,000 ft down $25\times3=75$, so start at 75 nm GS = 320 kts => $320\div2\times10=1,600$ => -1,600 fpm is your desired rate of descent. How do I calculate without using tangents for degrees, other than 3: 2,5; 4; 5 ...? In my last question I got it wrong, even though through math the answer was correct.

• Primary target: An aircraft not reporting mode-C, the only thing the controller has is the return on the radar. When a controller reports a primary target as traffic to other aircraft, the controller does not have the altitude of the target. Given this, I conclude that ATC radar does not have the altitude (angle-up) to the target, and only provides azimuth. So then without the altitude, how does the radar-system know where to put the target laterally on the screen? Example, a radar picks up a target that is 10 miles from the station. If the target is 0 AGL, the proper position would be 10

• This is what I know: $V_1$ is the takeoff airspeed after which the aircraft must take off, no matter what happens after $V_1$ has been reached. That's the easy part (I think). $V_R$ is the rotation airspeed Are there any other $V$-speeds? What I'm specifically curious about: Is $V_1$ related to runway length? Is there an absolute maximum $V_1$ for each aircraft type? If so, can it vary based on conditions (takeoff weight, density altitude, etc.) Are there circumstances where $V_1$ is higher than $V_R$? If so, does that mean that $V_1$ is never called on a normal takeoff

Data information