The Big Sky
An en route air traffic controller utilizes many types of airspeed to control with during the course of a day... ground speed, indicated airspeed, true airspeed, and Mach numbers to name a few. Different situations call for different scales; at high altitudes (Flight Levels) Mach numbers are common, while indicated airspeeds are used below FL240. True airspeed, which is filed in your flight plan, is utilized by the ERAM/HOCSR/HOST ATC computers to determine a ground speed on flight strips (although these are rarely used now, if at all) and in flight data processing that is computed using forecast and actual upper wind values. Ground speed is also read off of the radar display, and gives a controller an up-to-the-minute tool with which to work with. A radar ground speed readout is an average of the past few updates of an aircraft's radar track. A groundspeed displayed on a radar screen is a very important and useful tool for a controller.
In the en route world, radar fully refreshes about once every 12 seconds. An aircraft's trend of acceleration or deceleration was/is difficult to gauge, as the ground speed displayed was an average from the past 3 or 4 values - or up to 48 seconds ago. The terminal radar systems gave faster refreshes, and better opportunity for precise radar speed data. When I retired from the Center, we had groundspeed readouts to a single knot; but when I began controlling in the early 1980's, groundspeeds were displayed rounded to the nearest 10 knots. I'm wondering how GPS information, with its second-by-second information update, will be integrated through the newest ERAM computer and change how a Center controller operates.
In the early 1980's, we once worked a light plane flying into the teeth of a strong westerly wind while traversing northern Maine. He actually had a negative ground speed! One of my fellow controllers reported this to the pilot, and after a while a descent was initiated to get out of the strongest winds and regain forward momentum. Our radar didn't show a minus sign next to the ground speed, as it was tracking the rate of change between target returns. When we ran out a vector line that showed where the aircraft was going to be in 8 minutes though, the track was opposite from the plane's filed route of flight. The flight strip showed a "G NEG", or negative ground speed after the filed true airspeed.
Speaking of slow, during the early 1980's there were a few types corporate jets, transports, and jetliners that weren't very compatible (airspeed-wise) with the majority of high altitude inhabitants. Routine cruise speed for a DC-8 jetliner was around (Mach) M.80, with Boeing 747s up around M.84. Air Force C-5 Galaxy transports normally operated at M.77. Most corporate jets were compatible with these airliner and transports too. Older Air Force C-141s cruised at a slower M.74, and some early Cessna Citation I and II jets were even a bit slower at around M.70, as was the British Aerospace BAC-146 jetliner. This equated to 40 knots or more difference in groundspeed - clearly a rapid "overtake" situation in ATC lingo. Inevitably, you'd end up changing the slower aircraft's altitude to accommodate the faster majority of aircraft behind it, whether they wanted it or not. Sometimes jets ran into fuel management issues as they were forced down at less efficient altitudes because of their speed. In the 1980s especially, the faster majority ruled, and got the "choice" altitudes. Nowadays, with additional altitudes available through the adoption of RVSM rules and improved altitude performance of most aircraft, few aircraft are penalized for their less than average speed. There are even some speedy turboprops, such as the Piaggio P-180 Avanti, C-130J, and P-3 that get up into the FL310-FL330 range and mix well with jetliners.
Using Mach numbers, you could realize a groundspeed adjustment of 10 knots with a change of M.02 or M.03. Many times, you could equal out ground speeds of two dissimilar aircraft by speeding up one jet by M.02 and reducing the other by the same. One of the restrictions in the use of Mach numbers for speed control is that ATC can't assign a Mach number to an aircraft maintaining FL390 or above without pilot's consent, presumable due to the narrow range between maximum and stall speed at those flight conditions. Another peculiarity about speed control using Mach numbers is that a pilot has a window of plus or minus M.02 leeway to be compliant with an ATC Mach number restriction.
There were lots of opportunity to be confused about aircraft speed performance in the 1980s too. When the speedy Citation X (Mach .92 maximum speed, but rarely used) came along, a few controllers would ask "are you a fast Citation or slow Citation"? You had to keep your wits about you and watch out for any aircraft that was flying into your sector with slower or faster than the average airspeed. Anyone remember Presidential Airways? Presidential operated BAC-146s and Boeing 737s into Boston, and at different times of the day a different airframe type was used. You had to be cognizant of your Presidential flight in your sector... once in a while a controller, based upon past practice, would begin sequencing a string of Boston arrivals by reducing the airspeed of a lot of trailing aircraft behind what he/she thought was a BAC-146... only to realize it was a much faster Boeing 737 a bit later on! Air Canada and its affiliates operated BAC-146s too, and depending upon the jet stream winds over northern Maine, you could be working a BAC jetliner heading westbound into the wind with a ground speed 100 knots slower than that of a Canadian Forces CP-140 (Lockheed Electra/P-3 Orion) turboprop heading in the opposite direction.
Down at lower altitudes, around 16,000 feet, we used indicated airspeeds to maintain spacing while feeding arrivals to busy airports like Boston and Kennedy International in New York. Boeing 727s used on the Boston to LaGuardia shuttle runs could fly as slow as 210 knots or as fast as 350 knots, giving us great flexibility to match groundspeeds with other aircraft. The pilots used to sound real happy when we'd instruct them to "go fast", or "fly your best forward speed", even though it was inefficient... but their tone changed when jet fuel costs rose to one dollar per gallon. Today it is even more critical at four dollars per gallon to be operationally efficient, using an aircraft's maximum speed isn't as common as in the "good old days". Another restriction on ATC is that between FL280 and 10,000 feet, a recommended minimum airspeed of 250 knots for turbojets is required, except in rare cases.
Using radar-derived and wind-affected groundspeeds for controlling was an important skill that every developmental controller had to learn. Part of your transfer of control briefing between controllers as one was relieved from a sector was what the upper winds were like. As an instructor, you'd quiz your student as soon as they sat down with you at a sector, and ask them what direction the winds were from and how fast they were. If you were to be a successful controller, knowing how to use different types of speed control, using groundspeed, Mach numbers, and indicated airspeed were critical concepts to master.