A Transavia Boeing 737-700, registration PH-XRX performing flight HV-6051 from Rotterdam (Netherlands) to Alicante,SP (Spain), departed Rotterdam's runway 06 normally, however, on climb out the Mode-S transponder did not transmit any plausible altitude information. The crew soon after takeoff requested to level off, was cleared to maintain 5000 feet. Rotterdam ATC advised they did not receive any altitude information from the aircraft, the crew apparently switched source of the transponder and asked "and now", but ATC again advised still no altitude reading was available. ATC queried about their altitude, the crew reported they "believed" to be at 6500 feet, they were flying on their stand by instruments which they deemed reliable. ATC inquired with military three dimensional radar who informed the aircraft was actually at FL110. The crew decided to divert to Amsterdam and requested runway 06 at Schiphol Airport. On approach to Amsterdam the crew explained they had a normal takeoff from Rotterdam, in the climb they received indications of unreliable airspeed and altitude errors on both captain's and first officer's instruments, they believed their stand by instruments were reliable however. Amsterdam approach also confirmed receiving no altitude readout whatsoever. ATC and crew used another aircraft operating in the vicinity and maintaining FL070 to verify that the aircraft's TCAS indications were still working correctly, the crew reported they were descending through 4400 feet according to their stand by altimeter set to the local QNH, at the time the military radar reported they were at 3200 feet. The crew advised no assistance was needed after landing with the current situation on board. The aircraft landed safely on Amsterdam's runway 06 about 35 minutes after departure from Rotterdam.

A replacement Boeing 737-800 registration PH-HXC reached Alicante with a delay of 2.5 hours.

The occurrence aircraft is still on the ground in Amsterdam about 24 hours after landing. Prior to the occurrence it had last flown on Feb 19th 2021.