Performance Differences

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This isn’t a story so much as it is background that may be of interest. You would be correct if you’ve begun to get the idea there’s a lot to learn in the process of becoming an air traffic controller. In addition to hundreds of location identifiers (the three letter airport codes you may be familiar with), there is a thick procedures book, there are sector idiosyncracies, airspace configurations, special operations, frequencies, etc., all of which you must know as well as your name to be able to function effectively in the system. One of the subtleties is in the area of aircraft performance.

For controllers in the tower it’s important to be able to tell an Airbus A-300 from a Boeing 767, or a DC-8 from a Boeing 707. They look very much alike, but if you’re instructing a flight to “follow the Airbus” you’d very much appreciate it if he did and not the 76 (pronounced “seven-six” in the biz) in front of it. It’s important in the radar room, too, as a climb clearance to a Boeing 757 can have entirely different consequences than a climb clearance to an L-1011. The sort of airspace you work in makes a difference, as well. Airplanes that are hotrods in the airport environment may very well be dogs at FL350. Others may still be chomping at the bit at FL390.

In my days in High Altitude (regularly from ’70 until ’76, then sporadically from ’85 to ’97 when I retired) we found that in terms of level flight that B707s, 727s, 720s, 747s, DC-8s (both regular and stretched), DC-10s, and Convairs (both 880s and 990s) all pretty much kept the same pace—there being no more than a couple of hundredths of a Mach number between them. You could let any one of those run in trail of the other for a hundred miles without being concerned about a rapid overtake. Gulfstreams and Jetstars would do nearly as well.

However, throw a DC-9 or a B737 in the mix and you’d better pay attention, as they ran a full .05 Mach slower. Moreover, the 73 couldn’t go higher than FL330 and the DC-9 couldn’t go higher than 35 (although by the late ’80s, newer models could). All of the business jets (other than the two listed above)—the Saberliners, the Hawkers, the Falcons, the Lears—were even slower, although only by a couple hundredths of Mach.

However, once the Arab oil embargo kicked in, the playing field leveled some as the companies reined in speeds a bit to save fuel—Mach .80 in the cases of the jets that could cruise faster. Although a tri-jet (B727) or 7-oh would still outrun a -9 (DC-9) or a 73 it took a little longer. The one big difference was the L-1011. Apparently (as explained to me by a Tri-Star crew), the aircraft had enough nose-up deck angle when throttled back to Mach .80 that it actually used more fuel than if it was flown at .85. So, they kept flying at .85 while every one else was now at least .05 Mach behind. Yeah, these are the sorts of things one needed to be aware of.

Descents were another thing. Back in the days of “the pump and bang” (as the guys at ORD used to call the wide open times of ATC in the ’60s and ’70s), it was not uncommon to have airplanes like -8s and tri-jets running at 390-400 knots while descending (I saw Convairs—880s or 990s—do 410). At sectors like PMM and FWA, you could run a pretty effective speed control problem using airspeeds in 20 knot increments and get the spacing needed for the Terminal sectors you were feeding. We had to use grosser differences (30 or 40 knots) on the West side because of the winds.

The biz-jets couldn’t/wouldn’t do those speeds, of course, but there weren’t that many of them in comparison to the air carriers, and often they were going to a satellite airport anyway (PWK or MDW/CGX) so they didn’t need to be sequenced with the ORD arrivals, but if they did, they were automatically second or third when the controller was making the decision of who goes first. And then there was Northwest. For some reason, they had a company policy to not exceed 320 knots. That made them automatically last (metaphorically) when working up a sequence.

Fast forward a couple of decades—in the late ’80s or so, all the airlines seemed to have adopted a 320 knot optimum speed in descent profile for their aircraft and the paradigm shifted subtley for controllers who began to think of their speed control window as something between 250 and 320 knots (we weren’t supposed to slow anyone lower than 250 knots in the center). And for the most part I was able to adapt to that, but I knew something that the younger guys didn’t—they could go faster…and generally would, if asked. So, I would still set the first guy at 370 or better and work back from there. It always amused me that the young guys were using a speed for a maximum that we used to consider a handicap that automatically made the user second or last.

When I got back from sabbatical, B757s and 767s were all over the place and cruised at altitudes formerly reserved for Learjets—FL410 and above. I remember being astonished when I got to ZAU to find B727s and 707s coming in from the West Coast in a block altitude of FL390/410 (apparently they were only certificated to FL400, which wasn’t a legitimate altitude in those days, so we assigned a block). At ZJX, we very rarely, if ever, saw an air carrier above FL350. Those altitudes were for Lears and Saberliners. 75s and 76s were a whole new ball game. And particularly in the case of the 75, they got up there in a hurry.

1011s didn’t, though. TWA used to fly them on the ORD-LAX route. Most air carrier jets could be level at FL350 by the Mississippi River or just a little west—certainly by IOW. I remember L-1011s not being level at FL310 when we turned them over to ZMP out by DSM, almost 250 miles from ORD. They could go fast, but they couldn’t climb for crap.

I could write for hours about these subtleties but I’m not writing a training manual for controllers—I just wanted you to get a flavor of the sort of minutiae it was necessary to absorb to be effective at ATC. We knew a lot and we were pretty proud of it, but it’s hard for the layman to grasp because how can you articulate the reams of information that one often doesn’t realize they’ve accumulated, yet incorporate it seamlessly in their work on a daily basis?

Worse, it took years to accumulate, I can’t get rid of it, and it won’t even get me a cup of joe.

Last updated: 20 January 2011