Aviation In Canada

Thursday, August 18, 2005

Aircraft Speed Limits

I'm not exactly sure what made me think of this, but I did a search for it this morning and found that CARs was available online at the Justice Department of the Canadian Government in a searchable format. Anyway, on with the post.

I've had several questions as a controller, most of them off the radio, about the item formally known as the "speed limit order" before CARs came into existence and Air Regs and Air Navigation Orders were the laws of Canadian aviation. The most common question was this: If ATC asks a pilot to "Keep his speed up", does this mean he can exceed 250 knots indicated below 10,000 feet? There are two parts to the answer.

The first part is found in CARs, 602.32. Here's a link to it:

This pretty much says "no". The fact that it is spelled out about the 200 knots below 3,000 within 10NM of a controlled airport and there are no such exceptions for the 250 below 10,000 point says it all. ATC has neither the authority to direct a pilot to exceed 250, nor can they authorize a pilot request to exceed 250 below 10,000. Period.

The second part is where we'll get a bit wishy washy. The simple fact is that ATC can't be proof positive that an airplane is being flown above 250 below 10,000. There are a number of reasons that come together to ensure we can't know. ATC sees only groundspeed on the radar, simply because that's all a radar can track is your velocity relative to itself, firmly fixed to the ground. Next, the difference between true and indicated. You may be at 9,000 feet, indicating 250, but your true airspeed may be in the range of 275 or 280, so your groundspeed would be in the same range -- with a caveat from the following sentence. Next is winds. If you have a 50 knot tailwind, your groundspeed will be much higher, even near sea level where the difference between true and indicated speeds are very small.

ATC can always make a pretty good guess about whether you're over the speed limit or not, but honestly, I don't think there are many controllers out there who care. If you exceed the speed limit and it helps us and other aircraft behind you, we likely won't give it a first thought, let alone a second.

I'll offer this, too, though I'm not 100% certain it's true so take it with a grain of salt. I've been told that the 250 below 10,000 regulation is not a VFR see and be seen issue. I'm told it's an ICAO directive more to do with reducing damage due to birdstrikes, rather than an issue of other traffic. Someday I may actually take the time to try to confirm that, but it won't likely be for a while.