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This week's topic:
Speed Limits Explored
Introduction
Exceptions
Taxiing
Indicated vs. Ground Speed
Tying it All Together
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 (summarized, not exact text):
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.
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 30 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 annoy others if you're going too slow end. It really comes down to airmanship. Remember to consider your aircraft's configuration of landing gear when taxiing. If your aircraft has a high center of gravity, you'll be more likely to lean into the turn if you taxi fast. If your wheel track (the spacing between your left and right main gear) is narrow, this could lead to a tip over, maybe a wing tip strike. 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, making it possible for the tail to be swung out during a turn, at worst, turning the aircraft right around. 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.
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 aircrafton 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 all are generally in the same unit, 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.
What do these have to do with the speed limit order? Most mean nothing since the speed limit order is based entirely on IAS. What I want to explain here is mostly to newer virtual 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. Only in high-end, modern aircraft does instrumentation have the capability to transmit IAS or TAS and other data to the ATC facility. Aircraft with all the bells and whistles like ADS-B out, Air Data Computers, and Flight Management Computers may be able to transmit much of this data and more, but for the most part, this is not used in provision of ATC.
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 250kts. 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. This faster TAS results in a faster Ground Speed, showing an aircraft at FL300 doing about 400 knots over the ground while the pilot is telling you the IAS is 250.
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. An aircraft 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 switching from a tailwind to a headwind as the aircraft turns. Remember that the pilot 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.
New virtual Air Traffic Controllers should keep all of this in mind when sequencing aircraft, as real-world controllers do. Such a change in heading affects the speed as the aircraft tracks over the ground relative to other aircraft. In the above example, when turning from downwind to base leg, the push form the tailwind is gone, and then the speed drops again when the aircraft turns final. This contributes to "compression" in a sequence: Two aircraft downwind flying the same airspeed with 5 NM spacing will turn into less as the first aircraft makes the turn to base leg, and then it will drop again as the first one turns final. Certainly, each aircraft would experience the same drops at the same places (assuming they fly consistently throughout the whole process), so the time interval will be the same, but since the leading aircraft will see the reduction first, time-wise, the second aircraft will gain on it until it makes the turn. This means that the 5 NM will be reduced at the base leg turn, and then again at the turn to final. What you started with downwind may not be enough on final, especially if the wind speed at the altitudes of these aircraft is high.
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 mo@xlii.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)