Why did the Flightaware groundspeed readout suddenly spike for this flight? My 18yr old son is currently in Barcelona - first time abroad and in a plane. Being an overprotective parent I thought I'd track his flight so I could see when he had landed safely = and save the data for him so he could see his journey.

Anyhow, I noticed weird spike in the flight graph (which I couldn't save). A sudden jump of 200mph in air speed - and equally sudden reduction - at around 15,000ft. No jump in altitude increase though. Also a rapid change in direction.

The flight was on Friday 3rd at around 7.18 am bst leaving from Bristol. Data copied below - any thoughts?

BST Latitude Longitude Course  Direction  KTS MPH feet   Rate
Fri 07:22:11 51.3043   -2.6938 254° West  306 352 10,900 2,338
Fri 07:22:37 51.2927   -2.7573 255° West  322 371 11,900 2,759
Fri 07:23:38 51.2674   -2.8943 360° North 503 579 14,900 2,804
Fri 07:24:24 51.2459   -3.0081 253° West  340 391 16,900 2,365
Looking into one set of data from the the popular Flightradar24 service it is not visible.
I think I found the data you're looking at. Since the heading also suddenly changed to (exactly) 360, I'm thinking this is just an invalid data point. Notice that the receiving station jumped between these measurements and something could have happened with the processing in between.
From the same site:

Aircraft with an ADS-B transponder broadcast their identification and this three dimensional position (latitude, longitude, altitude)

This simply implies that this service tries to interpolate the speed from the time between two positions. So say that the clock at the receiving station or something is off (for instance, just an idea), it will just screw up the reading. It comes with the disclaimer:

I believe I witnessed a traffic conflict, altitude deviation, or some other anomaly. Should I report it?
Please don't. Rather, read the terms of use and understand that this data is for casual observation only and not for any operational purpose. FlightAware technology was not designed with the intent to observe safety or regulation anomalies, so please do not try to use it for that purpose.