Wind calculation

[dstclair]

I flew down to Biloxi this weekend at an altitude that had forecast winds (for part of the trip) in 40+ kts range that would be most a crosswind. The first 2+ hrs everything worked fine -- winds and OAT were aligned with forecast as per my D100.

When I hit the part of the flight that was to be in the heavier winds, I gradually saw an increase in the magnitude. OAT still read what seemed correct but I started seeing winds in 70-95 kts range as a crosswind on the D100. The winds were variable in strength and direction with light/occassional turbulence.

I don't think the winds were actually THAT strong. OAT was in the 28-32 F range. Pressure was dropping fairly fast as I went through the cold air mass -- going from around 30.4 down to 30.0 over a 100 mile stretch. Wind did seem to change directions by 30-45 degrees at times. The calculated wind tended to stay above 50 but would drop down to ~15 for awhile.

Was the calculation just due to changing conditions?

D100 worked as expected on the 4.5 hr return flight.

 

[dynonsupport]

Wind questions are always tricky to answer. The base answer is that the wind calculation takes GPS, mag heading, IAS, TAS, OAT, and crunches them just like you would on your E6B to come up with an instantaneous wind number. Any anomalies in those readings will cause erroneous wind reading, and unfortunately it's often not trivial to figure out which of the parameters are wrong.

The wind calculations are temporarily suspended when you're turning, so you might see it change after maneuvers. In turbulence, you might see inaccurate numbers as well due to the instability of some of the factors used in the calculations.

 

[skysailor]

Remember the quick rule of thumb is that for every 60 knots of true airspeed you cruise, one knot of crosswind equals one degree of heading crab to track course for the crosswind. As an example, if your plane cruises at 180 KTAS and it takes 15 degrees of heading to the right to maintain your ground track, the wind has a 45 knot crosswind component from your right. If you were seeing the corresponding crab angles, the wind was there. High and variable winds are common with frontal passage.

 

[dstclair]

Well, I've got around 120 kt cruise and was doing 45 degree of crab to hold course which equates to 90 kts of crosswind. That's assuming the DG tape was correct. No reason to believe otherwise. Guess all is working fine.

BTW -- the Dynon AP worked pretty well through the heavy crosswinds.