Airspeeds & Mach

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More subtle but necessary information. Down low, say three or four thousand feet MSL (above mean sea level), the different airspeeds of consequence in flight are virtually the same. If one’s airspeed indicator is showing 200 knots, then you’re essentially moving through the air at 200 knots. Problems arise, however, when we go higher because of how airspeed indicators work and the laws of physics. There are other distractions, as well, all of which we must have a notion in order to be effective controllers.

An airspeed indicator is basically a pressure gauge. It compares the ram air (collected at the pitot tube) to static air (the the pressure of the atmosphere surrounding the airplane wherever it is) and displays that difference on a gauge in the cockpit (flight deck) which has a scale calibrated as airspeed. If no airplane ever left the ground, we could use that number as an absolute and you could stop reading this article. However, a fundamental law of nature is that air pressure decreases as we get higher and higher above the ground. In fact, it’s a fairly constant change in pressure (a principle on which another airplane gauge, the altimeter, is based) and is considered roughly one inch per thousand feet. What’s roughly 15 lbs/inch² at sea level is about 10 lbs/inch² in Denver (elevation about 5,000 feet).

The thinner (less dense) air at altitude means not only is there less to measure, but the ratio of ram to static is changed, as well, and the 200 knots we saw at sea level is now somewhat less, even though we’re still going about the same actual speed. Thus we wind up having a couple of different speeds to consider, since they each have relavence in different ways.

The airspeed read directly from the airspeed indicator we call “indicated” airspeed (IAS). It’s useful because it’s a directly measurable speed available in any airplane. And, at the same altitude, two airplanes indicating the same speed are truly going the same speed. That’s a very valuable property of which we can make considerable use in ATC.

However, the actual speed at which you’re moving through the air isn’t the same as IAS, and becomes less so the higher you go. And if you have any hope of accurate flight planning, you must be able to apply “true” airspeed (TAS) to your calculations. While there is a factor which you must to apply IAS to net true airspeed, it’s best done either using the old E6B type of aviation calculator, or an electronic calculator. It takes into account IAS, altitude, and atmospheric pressure to net TAS. Also an important property for ATC, but generally not past the flight plan processing stage.

Down low (perhaps below the Flight Levels—that is 18,000 feet) airplanes can be flown pretty well by use of IAS (the standard expression of speed in ATC is in knots, and the abbreviation “K” is applied to the speed abbreviations netting KIAS and KTAS). However, once in the upper reaches, the disparity between KIAS and KTAS becomes too much and another factor becomes important—that is the relationship between true airspeed and the speed of sound. There are aerodynamic issues which become limiting to aircraft design as we approach the speed of sound. Also, since air becomes less dense as altitude is increased, the speed of sound actually decreases. Knowing the ratio of KTAS to the speed of sound is thus important, too, and fortunately, is expressed as a Mach number (the ratio as a percentage).

There is one final variable to apply to speeds, and that is the effect of winds aloft. One can’t just point their aircraft toward a destination 200 (nautical) miles away and expect that their 200 KTAS will get them there exactly one hour later. When (not if) they encounter winds aloft it will increase their time in the air if heading into the winds, or they’ll get there sooner if the winds are behind them. The actual speed one makes over the ground is called, not surprisingly, ground speed. Ground speed as a specific quantity has no significance to ATC, but that there is a lot of it or very little of it, might. Moreover ground speed for flight planning is very important because on long flights, intermediate fuel stops may be necessary, and where they might be made is predicated on the real, over-the-ground speed the airplane is making, but that is a pilot effect, and not one for ATC.

To tie this altogether, let’s consider two aircraft—one at FL370 filed at 480 KTAS and one at FL250, also filed at 480 KTAS. Let us say they’re one right above the other—one target on the radar. Here are some interesting factors (we’ll ignore wind for now):

With the identical TAS (and assuming no wind) those two airplanes are going to stay one atop the other for quite a while. But we have to get them down and intrail for the terminal arrival sector. How are we going to pry them apart?

First, we need to throw in a rule here—we’re not permitted to slow aircraft below 250 KIAS (I don’t remember the exact parameter, but it pretty much applies throughout ARTCCs). There are a number of ways, and depending on other traffic, different choices might be better than what we’ll illustrate here. However, the most obvious way is to take advantage of the disparity between IAS and TAS. 250 KIAS isn’t going to make much of a dent in Ocanic’s TAS. That might not matter if we’re sequencing for a terminal 400 or 500 miles away, but we need to have this resolved in 150 miles.

So, we can slow TransGlobal to 250, which is about an 80 KIAS reduction. We only need ten miles in trail, so we probably only need the reduction for about 50 miles. Of course if we have 100 to work with, we don’t have to be quite so Draconian, and we can make the speed reduction to 280 KIAS. In any event, with this reduction in hand, we can start Oceanic down in a bit, but we need to assign him an airspeed so he doesn’t slow down on his own. Hmmm. We don’t want him to keep his present speed—275 KIAS—as he’ll never pull away from TransGlobal. And TAS has no meaning in the cockpit—in other words, you can’t read it off a gauge. We could just say “keep your speed up” and we did that for years, but it is ambiguous, because one pilot’s keeping it up is very often quite different from another’s.

The solution is in the Mach number. First of all, there’s a Mach meter in the cockpit. Second, Mach is a reasonable approximation of the TAS we want to achieve. So, we can assign Oceanic to “indicate .83” on the way down. Woops, there’s another problem. At some point, aircraft have to transiton from Mach number to KIAS due to airframe limitations in the lower, denser air. That transition will probably be at about FL200, but there’s no reason to not include it in our thinking now. Ultimately what we really want is a hard KIAS that we can pass along to the Terminal controller—something on the order of 320 KIAS. So, we tell Oceanic to “indicate .83 until reaching 320 knots” and that will ensure he flies a pretty consistent TAS which you need to make your speed control problem work.

When TransGlobal falls sufficiently in trail for your problem you just increase his speed to 320 KIAS and Bob’s your uncle—you’ve just run your first ATC sequencing problem.

I was aware early on—in fact back in my flight training in ’66—of the relationship between IAS and TAS. Naturally it was easy to understand once I got into ATC, although at ZJX we didn’t run that many speed control problems, and even when we did, the highest airplane was usually at FL350 where the IAS wasn’t quite so low. The significance of the Mach number came to me after I’d been at ZAU for a while, and although I’m sure I didn’t invent the idea, I certainly developed a usable technique for it well before it became a legitimate procedure. Let’s just say I was an early adopter.

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Last updated: 21 April 2009