Recently I took a trip to the US, which offered a good opportunity to test my SensorTag altitude logger by recording a profile of the whole journey there. The trace revealed some interesting details about the flights I took, and airline operations in general.

Here’s the profile for the entire trip:

Full trip profile logged on the TI Sensortag.

The x-axis shows elapsed time in minutes. The altitude is shown in metres, measured relative to the start of the trace (not too far above sea level). Despite that I’ll be using feet as the unit of altitude here, since that’s the standard used in aviation. Because the logger calculates altitude based on air pressure, it is affected by cabin pressurisation. Instead of recording the true altitude of the aircraft it gives a trace of the effective altitude inside the cabin.

The first big peak at the blue cursor is a flight from Edinburgh to London Heathrow. Comparing the cabin altitude trace against real altitude data makes it easier to pick out the main features, so here’s a chart showing this flight’s altitude as broadcast over ADS-B:

Chart showing EDI-LHR flight’s altitude as broadcast over ADS-B.

And this is a closeup showing what my altitude logger recorded for the same flight:

SensorTag record for the same flight [EDI-LHR].

The cursors mark where I think the flight started and finished, based on the fact that the plane was in the air for 70 minutes. From takeoff the pressure falls steadily until the effective altitude in the cabin is about 7000ft, at which point the aircraft is actually at 37000ft. After cruising there for 12 minutes the plane descends and cabin pressure steadily increases.

The cabin pressure reaches ground level before the plane actually lands, so the trace stays flat for the next 12 minutes. In fact, this section of the trace is effectively below ground level while the plane approaches landing. The plane’s environmental control system has deliberately overshot and pressurised the cabin to higher than ambient pressure at the destination. At the orange cursor marking the end of the flight you can see a slight increase in altitude. This is when the flight is over and the controller opens the pressurisation valve to equalise with the external air pressure.

It seems this extra pressurisation is done before takeoff and landing to help the system maintain a steady pressure. There’s a detailed explanation of the reasons for this here: http://aviation.stac…e-on-the-ground

Now on to the second flight, which was from Heathrow to Dallas Fort Worth. First the ADS-B trace:

Chart showing LHR-DFW flight’s altitude as broadcast over ADS-B.

And the altitude logger’s version of events:

SensorTag record for the same flight [LHR-DFW].

Again, the cursors mark the start and end of the flight and line up with the reported duration. The “steps” along the top of the trace match up with changes in cruise altitude from 32000‑>34000‑>36000ft. Maximum effective cabin altitude is about 5500ft, lower than the first flight even when the lower cruise altitude is taken into account. I think that’s down to the use of a newer 777 on the international flight compared to the A319 on the domestic route. Modern planes are increasingly designed to offer lower effective cabin altitudes for passenger comfort.

The stepped flight profile is used to maximise fuel efficiency. Flying higher reduces losses to air resistance, but early in the flight the aircraft is heavy with fuel and climbing is expensive. As the fuel is burned off the optimal cruise altitude increases, so ideally the plane would climb to match. In fact the plane can’t climb gradually because modern air traffic control regulations restrict aircraft to set flight levels. The best option under these restrictions is to perform a “step climb” up to a higher level when it’s more fuel-efficient than the current one. The flight levels are multiples of 2000ft for flights from the UK to the US, which is why the steps are 32000->34000‑>36000ft.

Wrapping up, one of the things I hoped to test by recording this journey was high rates of altitude change. The altitude logger can currently handle rates of change up to ±127m/s, but I don’t get anywhere close to that in normal use. It turns out the rate of climb and descent for this trip was really quite tame; sudden changes in cabin pressure would be very uncomfortable after all. I think I’d need to take the logger skydiving or bungee jumping to get viable test data for this case! I’m now considering dropping the sample rate to 1Hz which will halve the maximum rate of change, but double the duration of log data that will fit in the flash memory (currently 67 hours).