2023-08 · NASA ASRS report 2029348
Fractional aircraft pilots reported a problem holding an assigned heading and altitude. Reporters stated GPS testing in the area may have contributed to the deviations.
While at FL310 and just given direct to DRK (from a heading of 040); our plane went direct to DRK; but then started turning to the right to compensate for winds. The winds started fluctuating between 35 and 200 plus kts (for about 5 minutes) direct crosswind out of the south. I pushed the TCS (Touch Controlled Steering) button to stop the continued right turn while watching the winds fluctuate. In all the confusion and distraction; we climbed 270 ft. high with traffic out ahead of us and coming toward us showing plus 800 [ft.] higher than us. I pushed the nose back over and descended to FL310. LA Center asked if we were given a climb. At the time; we were in a possible GPS testing area from the BVL VOR from XA09 till XB29 (but it seems it was probably a little to far to the north to affect us). We're not sure if that contributed to the plane turning to the right. We received no indications of a fault of any sort. I should have left the autopilot on and gone into heading mode until the winds were figured out.
We were operating a flight on Aircraft X from ZZZ to ZZZ1. We were between the LANCY fix and DRK on J134 at 31;000 ft. waiting for a climb to our filed altitude of 41000 ft. We were flying an assigned heading of 040 for traffic. We then were given direct to DRK. I entered the direct and as I was monitoring; I noticed the winds in the FMS were wildly fluctuating anywhere between 27 kts to 200 plus kts from the south. The winds that day were really around 27 kts. The FMS was thinking we had a 200 plus kts crosswind and turned 45 plus degrees from our true course to ZZZ into the wind. I brought all of this to the attention to the PF (Pilot Flying) as I was noticing it. He agreed that something was wrong and as I was trying to figure out what was going; on the PF TCS'd (Touch Control System) the controls to stop the turn. At that time; I noticed the aircraft began a slight climb and I brought that to the attention of the PF. At one point; I mentioned we have to stop climbing and get back down to 31000 ft.I believe we ended up anywhere from 250 to 270 ft. high. LAX Center said he saw us turning way off course and then queried us; Have you been issued a climb clearance?" At that point; I notified ATC we did not receive a climb clearance and he asked what our altitude was. At that point; we were heading back down through 31;100 ft. and I advised him of that. We also informed him our FMS winds were acting strange causing the extra turn. There was traffic off of our left and I noticed the TCAS said they were above 800 ft. during the TCS maneuver. This whole event lasted about 4 minutes in total. After returning to 31000 ft. and back in the general direction of DRK; the FMS winds settled in to the correct appropriate winds between 25 to 35 kts. There was some GPS testing at the time at BVL that ended at XA29 and our event began around XA25. After that event; no other issues happened with the FMS/GPS for the rest of the flight. Both pilots now believe if something like this happens again; to put the aircraft into HDG (Heading) mode and not TCS mode to prevent any altitude gain or loss."
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.
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