# How is taxi speed measured?

What is the maximum taxi speed and who defines it? has a lot of interesting thoughts and facts about what maximum taxi speed is admissible or practical. It brings me to this new question: how does a pilot know how fast he/she is going when on the ground, apart from just looking out the window?

Afaik most planes only have an airspeed indicator, which I suppose is not of much use while taxiing due to surface winds being in the same order of magnitude as ground speed. Also I wonder if an ASI has any usable accuracy near the bottom of its range. The same might be true of GPS speed indication.

Did some more research and found out GPS is much more accurate than I thought it was. http://www.ncbi.nlm.nih.gov/pubmed/15519597 and other sources find it better than 0.5 ms-1 most of the time. I guess that is better than what can be calculated from wheel speed, especially with big, fast wearing tyres.

• On old 747-100/200 the ground speed was shown on the inertial navigation systems, and it was accurate at taxi speeds. These days I'm sure the glass cockpits can display the ground speed. It would certainly be accurate just as your car's GPS readout is accurate at low speeds. Looking out the window can be a surprisingly INACCURATE way of judging groundspeed, especially if a pilot has just transitioned from an aircraft where they're close to the ground to one where their eye level is over 30 feet up. Aug 16 '16 at 20:52

Some aircraft have a "taxi speed indicator". Here's one I found on ebay for an Airbus:

• Tops out at 20 knots, huh? Good thing Southwest doesn't operate Airbuses -- they'd need a much higher top end than that! Aug 16 '16 at 21:00

These days in most aircraft, GPS units are standard. You can read your ground speed directly on the screen, which is really helpful when taxiing if you want some extra info about how fast you're going. Try to keep it under 20 kts! This image is from my Garmin GNS-530W GPS unit in the air, but the display is the same on the ground. You can see the ground speed as GS in the bottom left corner (where it says 163 kts).

• These days in most aircraft, GPS units are standard. <-- Citation Needed! (seriously, I can walk down the ramp and well over half the aircraft don't have a panel-mount GPS). They're becoming more common, certainly, but they're far from ubiquitous. Of course in 5 years time I'm pretty sure that statement will be correct :) Aug 17 '16 at 17:33
• @voretaq7 fair enough, old ramp queens and J3 Cubs don't have em, but most anything newer than 1990 does. Don't think I'm ripping Cessna 180s either - but I meant what I said - "these days"
– Pugz
Aug 17 '16 at 17:36
• @Pugz, 1990? I don't think so. IIRC even some large jets only added it around 2000 when SA was disabled. Aug 24 '16 at 19:34
• @JanHudec I guess I'm new school :-)
– Pugz
Aug 25 '16 at 22:32