Mini flight reports:
Redhill to Lydd, with GPS data

This section features miniature accounts of some of the flights I do. You can use the following links to jump to other reports:

Redhill to Lydd, with GPS data

22nd March 2003

Today I persuaded Andy Coyte, a fellow pilot based at Redhill, to come flying with me to Lydd in G-BSEP. As well as being a pilot, Andy is a fellow software engineer and an incurable gadget freak.

For today's flight, Andy strapped himself in and then set about installing his various toys: hand-held GPS unit stuck to the windscreen, iPAQ on his lap connected to the GPS unit via a serial cable, to capture the NMEA messages containing our position and height information. NMEA is a standard format for text-based serial messages that was originally developed to enable electronic devices on ships to exchange information such as position, course and speed, heading, etc. For more information about NMEA, visit the NMEA web site.

Finally, out came Andy's rather impressive new digital camera to take some pictures during the trip. Andy has also written a computer program to plot the GPS position data graphically, so here are the plots for our flight to Lydd...

GPS lat / long track

Plot of lat/long track between Redhill and Lydd, showing a nice rectangular circuit pattern at the end.

The image above shows a plot of our lat/long position during the flight to Lydd. Starting at the top-left corner of the diagram, you can see that we had to fly approximately due east because of the controlled airspace around Gatwick (see the image below). When we reached the edge of their zone, we were able to turn and fly roughly south-east towards Lydd, making occasional turns to take in the views as we went.

Anybody who is unfamiliar with aviation procedures might be a bit surprised by the curious rectangular shape that we flew before landing at Lydd. This is known as the "circuit", an invisible rectangular pattern that all traffic follows when in the close vicinity of an airfield. You can think of this as being like a roundabout on the roads, where traffic can join the pattern at various points with the ultimate intention of making an approach to the active runway. The purpose of the circuit is to make sure that everybody is flying in the same direction - in this case it was a "right-hand" circuit where everyone flies clockwise making right turns. This imposes a degree of order on the traffic flow in the airfield's immediate vicinity, and helps pilots to anticipate each other's intentions.

If you look very closely at the end of the line, you can just see the U-turn that I made off the runway into the parking area.

The image below is taken from my chart, and shows the controlled airspace in the vicinity of Redhill aerodrome. We are right on the northern edge of Gatwick's controlled airspace - in fact Redhill's ATZ lies partially inside Gatwick's zone. The red line shows the path I took out along the railway line underneath the Gatwick CTA (shown with a big blue border on the chart), before turning south-east when I was abeam Bough Beech reservoir. You can also see the London TMA marked in red, which explains our 'stepped' climb to 2400 feet and then to 3400 feet.

Picture taken from an aviation chart, showing the controlled airspace around Redhill aerodrome.

GPS height data

Provided the GPS unit can acquire enough satellites, it is capable of deducing your height above sea level. This information is also contained in the NMEA messages coming out of the GPS, and Andy's computer program can plot a graph of the height against time, as shown below...

Plot of GPS height between Redhill and Lydd, showing the 'stepped' altitude that we followed underneath the controlled airspace that surrounds London.

There's a tremendous amount of detail in this graph. For the first fifteen minutes or so, nothing much happens - that's when I'm running through the pre-flight checks and engine run-up. The take-off is fairly obvious, and then you can see that we climbed in several stages during the flight. The reason for this is the presence of controlled airspace above us - first the Gatwick CTA which starts at about 1500 feet, and then the London TMA which initially starts at 2500 feet and later steps up to 3500 feet as we travelled further south-east. As the graph clearly shows, I generally like to fly as high as the airspace allows, usually with just a 100 foot margin to allow for slight ups and downs caused by thermals. In fact, the TMA would have allowed me to fly higher at times during today's flight, but there was little point in climbing above about 3400 feet because we had to make a descent into Lydd anyway.

Finally, you can see that the height decreases to 1000 feet, which I maintain whilst flying around the circuit, before making the final approach to land.

If you look back at the take-off climb, you can even see a tiny 'dip' where the aircraft climbs slightly less for a moment. The reason for this is that I made this take-off with 10 degrees of flap to shorten the take-off run. At about 300 feet, I removed this flap setting, which causes the aircraft to lose a small amount of its lift momentarily until it accelerates to a slightly faster climb speed.