Aviation Topic of the Week
By Michael Oxner, April 13, 2003


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This week's topic:
Speed Limits Explored

Introduction
Exceptions
Taxiing
Indicated vs. Ground Speed
Tying it All Together

Introduction

In the early days of aviation, there was no such thing as a speed limit. In response to safety concerns, the International Civil Aviation Organization, known as ICAO, adopted a standard for a speed restriction that relates to traffic, and also to other hazards such as birds in flight. I'll attempt to explore a little further.

The Canadian Aviation Regulations (CARS) define the only existing speed limits for aircraft in Canada. CARS 602.32 states that no person shall operate an aircraft in Canada:

  1. Below 10,000 feet ASL at an indicated airspeed of more than 250 knots; or
  2. Below 3,000 feet AGL within 10 NM of a controlled airport at an indicated speed of more than 200 knots unless authorized to do so in an air traffic control clearance.
Exceptions

There are some exceptions, though. If you're on departure, you can exceed the 200 knot limit. If you're so authorized by the Transportation minister, you can bust it. If the minimum safe airspeed, given the aircraft configuration, exceeds either of these speeds, you can exceed the limits, too, but you can only fly as fast as the minimum safe speed for the aircraft configuration while below 10,000 feet. Even the SR-71 and F-104 could fly below 200 knots, though, so this exception really wouldn't mean much. Especially since the military doesn't always heed these restrictions anyway.

Taxiing

Technically, there are no speed limits on the ground for taxiing aircraft. It is good practice to taxi relatively slowly to allow yourself reaction time to everything from ground movements of other aircraft and service vehicles to animal/bird activity and even things like gusts of wind if you're in a light aircraft. My flight instructor some 15 years ago said, "you know you're taxiing too fast when your airspeed indicator needle comes alive." He didn't intend to allow me to taxi my Cessna 150 just below the 40 knot lower limit, though. Generally speaking, a speed of 10-20 knots is recommended, although you may be seen as a granny behind the yoke if you're going at the slow end of that scale. It really comes down to airmanship. Remember to consider your aircraft's configuration of landing gear when taxiing. If you're running a high-wing aircraft, you'll tend to have a high center of gravity, and if you're wheel track is narrow (spacing between your left and right main gear), you'll be more likely to lean in the turn, especially if you taxi fast. Also, taxiing a tail dragger can be more of a challenge at higher speeds than an aircraft with "tricycle" gear with the possibility of a "ground loop" coming into play. For those who don't know, a tail-dragger has it's center of gravity between the tail and main wheels. If attention is relaxed, the tail may stray far enough to the side to allow the center of gravity to "spin" the aircraft around like a grocery cart with a bad wheel. Taxiing slower will help give you more reaction time to such an event if it starts.

Indicated vs. Ground Speed

So what's the big deal here? 250 knots is 250 knots, right? Well, sort of. There are many measures of aircraft speeds, and many of those are reported in knots, too. The one we should all be seeing and flying our aircraft on is Indicated Airpseed (IAS). This is quite simply the value the needle points to on our airspeed indicator. At low altitudes, the aircraft's Indicated Airspeed is very close to its True Airspeed (TAS). True Airspeed is the aircraft's actual speed in relation to the air around it. Another speed of significance, during navigation in particular, is Ground Speed. This is the aircraft's speed made good over the surface of the earth. All three are generally measured in knots, but most often expressed in the same unit as each other, whatever that may be. There are others, like Calibrated Airspeed and Equivalent Airspeed, as well as Mach Number and so on, but I'll leave these for another discussion if the interest is there.

What do these have to do with the speed limit order? Well, nothing really, since the speed limit order is based entirely on IAS. What I want to explain here is mostly to newer ATC who don't quite understand what they're seeing on their radars. Radars track aircraft by comparing the last known position to the position of the current radar return, calculating track and speed by measuring the distance between the positions (and typically averaging this value over a few 'hits'). This means a Ground Speed is reported in the data tag on a radar display, and also that you're observing "Track" made good over the surface of the earth instead of "Heading", as well. Currently, standard instrumentation does not transmit IAS or TAS to the ATC facility.

Indicated airspeed is more accurately a measure of the strength of aerodynamic forces acting on an airframe than it is for an actual speed. Yes, our Airspeed Indicator (ASI) is calibrated to read in Knots (or MPH, or even some in km/h), but it really has little to do with actual speed. As you climb higher and higher, the air thins out, reducing the aerodynamic effects for a given true airspeed. If your aircraft will cruise at 250 knots indicated airspeed at sea level, it will likely be very near that at 10,000 feet, FL200 or even at FL300. The fuel burn will likely be very similar at all these altitudes since you're pushing the same amount of air to get your ASI to show 250 kts. The difference will be the True Airspeed. At sea level, 250 Knots Indicated Airspeed (KIAS) is about 250 knots TAS. At 10,000 feet, 250 KIAS is about 290 KTAS. At FL200, the same 250 KIAS translates to approximately 340 KTAS, and it approximates 400 KTAS at FL300. So even though you're pushing the same amount of aerodynamic forces at each of these levels, the thinning air means you're flying faster than you're airspeed indicator shows.

Tying it All Together

I know, I know, I'll get back to the speed limit order. Remembering that 250 kts indicated at 10,000 feet comes out to about 290 knots true, in still air the aircraft's ground speed will work out to match that value. But how often, really, do you get still air at 10,000 feet? The winds may have an amplifying effect on the aircraft's groundspeed. For example, take an aircraft flying at 250 KIAS at 10,000 feet. He's "truing out" at 290 knots. Now add a 40 knot tailwind. The aircraft flying at 250 KIAS is now doing a groundspeed of 330 knots. It looks to the uninitiated ATC as though the pilot is busting the speed limit order. As this aircraft descends, the winds are likely to back off a bit, and the difference between IAS and TAS will also diminish. Eventually, 250 KIAS will look more and more like 250 kts groundspeed. But you'll still have the effects of wind, even when TAS and IAS are roughly equal. He may fly downwind and ground 270 knots, and then turn into the wind and ground 230. This would demonstrate the effects of a 20 knot wind. Remember that he needs your ATC authorization to exceed 200 KIAS below 3,000 feet within 10 NM of a controlled airport. And remember that a controlled airport is one with a tower in operation. An FSS at the airport means uncontrolled, so the 200 knot speed limit doesn't apply. Typically in VatSim, we consider it a controlled airport if the arrival controller, or center controller, is working his own position and controlling airport traffic at the same time. So a pilot flying below 10,000 feet with a radar data tag showing something above 250 knots for a ground speed may not actually be violating this speed limit.
 


Another topic down? Clear as mud? Feedback is good from my perspective. Whether you like this page or not, feel free to e-mail me at moxner@nbnet.nb.ca to let me know. As always, if there is anything you don't agree with, or don't understand, I'll attempt to clarify anything you see written here. There are no stupid questions but the ones not asked. (I used to believe that until I had kids and heard some of theirs, but I'll say it now anyway)