I am wondering is there any effect from wind on the altimeter reading?
I think when the wind is blowing, the static pressure around the aircraft in flight will somehow change and this can impact the altimeter reading. Is it correct or wrong?
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7$\begingroup$ Steady wind has no aerodynamic effect on an airborne aircraft in any way. "Wind" is the motion of the airmass relative to the ground, but the aircraft isn't connected to the ground. The aircraft is physically connected only to the airmass. $\endgroup$– Dave-CFIICommented Nov 28, 2023 at 15:03
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3 Answers
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A steady wind will not effect the altimeter reading on an airborne aircraft in any way, since the aircraft will be moving in relation to the airmass. This fully removes any effect a steady wind will have on the airframe and pressure sensitive systems attached to it.
Gusts may have temporary effect on altimeter reading depending on the location of the static port, a sudden gust may alter the pressure distribution around the airframe for a very short period of time inducing a higher or lower than actual pressure reading.
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3$\begingroup$ Not only can the actual pressure distribution change, but the wind gust can blow into the static port and thus show the dynamic pressure caused by the gust wind velocity instead of the actual (=static) pressure. That is actually a real problem when measuring turbulent pressure fluctuations in turbulent environment, the sensors will pick up a lot of noise from the dynamic pressure. $\endgroup$ Commented Nov 29, 2023 at 13:34
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Yes, if a gust of wind was blowing directly into a static port, it could cause a temporary increase in measured static pressure and a drop in the altitude readout.
Traditionally this was corrected by putting static ports on both sides of the aircraft and connecting both of them together, in a T layout. This reads the average pressure on both sides.
Modern large aircraft instead use digital sensors on both sides and the air data computers combine the data. This eliminates potential leakage in the piping, which has caused gross altimeter errors in the past.
Large aircraft combine data from multiple sensors, particularly accelerometers from the inertial nav system, and AOA and sideslip probes. These allow corrections to be made for gusts and local airflow, important to be able to meet altimetry accuracy requirements for RVSM and Baro-VNAV. Another trick is that at least one sensor will be placed asymmetrically, so that vertical gusts will have a unique signature.
This is why air data functions are in the same box as the inertial reference units, forming the integrated air data inertial reference system (ADIRS).
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static pressure around the aircraft will somehow change, and this can impact the altimeter reading
At the worst possible moment, when trying to hold a ground track in a landing pattern.
When holding a ground track, effects of wind direction can produce a relative wind that is not aligned with the direction of flight.
This could produce a wind sideforce on the aircraft, potentially affecting the static tube pressure by increasing (windward) or decreasing (leeward) "static" pressure.
Designers solve this by placing static ports in areas less affected by sideslip and/or gusts, and some, like the Cessna 172, have more than 1 static port, giving an average static pressure.
Any blockage, such as by icing, could also play havoc with the altimeter.
The Cessna 172 has 2 static ports, on either side of the fuselage, near the engine cowling.
answered Nov 28, 2023 at 19:20
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$\begingroup$ Re "When holding a ground track, effects of wind direction can produce a relative wind that is not aligned with the direction of flight."-- not if a) the wind is steady in speed and direction (and also is steady across the altitude spectrum, if the plane's altitude is not constant), and b) the pilot initially gets the plane onto the proper ground track by making a coordinated turn as needed until the ground track is correct, and then levelling the wings and relaxing all control input and trimming for normal coordinated flight. Sure, gusts can create transient side-wind components. $\endgroup$ Commented Nov 29, 2023 at 0:37
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$\begingroup$ But, just to be clear, I didn't DV (yet! ; ) ) $\endgroup$ Commented Nov 29, 2023 at 0:40
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2$\begingroup$ A side slip approach holding the nose in line with the runway is a case to be considered. Gusts and changes in wind are also more prevalent closer to the ground. Any how about slipping with an engine out? Yes, we would all like to be coordinated in a smooth steady wind, but it isn't always that way. $\endgroup$ Commented Nov 29, 2023 at 9:05
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1$\begingroup$ That close to the ground, the pilot shouldn't be too fixed on a pressure altimeter (allowable error, 75'). A radio altimeter, maybe - for Cat II or Cat III ops. The question is only about effects of wind, not slipping, engine-out, etc. $\endgroup$– Ralph J ♦Commented Nov 29, 2023 at 16:12