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“Affirm”
This shortened version of “affirmative” simply means “yes”. Yes was too short to be always cleary understood over radio, and “Affirmative” was too similar with “Negative”, so “Affirm” was created.

There are many reasons for which ATC can not always answer positively to pilot requests. Traffic, airspace restrictions, weather, worload, … but controllers do their best to anser with “Affirm” as often as possible. As pilot, I also do my best to answer ATC requests with “Affirm” as far as practical, to also help them, and to keep the system as smooth as possible.

“At your discretion”
This is kind of an extended version of “Affirm”. I can’t remember hearing it while flying VFR. I can imagine it for a pilot declaring an emergency, and being clear to land on “any runway at his discretion”.

“At your own discretion” is the perfect answer when and IFR pilot requests to deviate from his route to avoid adverse weather. We often request to deviate by XXX degrees to the left or to the right, to give the controller an idea. When controller answer such requests with “Turn is approved at your discretion”, it means that the pilot gets the freedom to turn as much as needed to circumnavigate the problem.

It is frequently accompanied by “Report when able to…”. Controllers know well that aircraft, and especially light ones, can not cope with any weather, or any problem on board. In case of emergency, controllers can also allocate a vertical slice of airspace to be used “at pilot discretion”.

“Direct to”
This one is definetly IFR oriented, but easy to understand for anybody. IFR routes are not always straight lines from A to B. There could be military zones, approach areas, or other restricted areas between A and B, but these zones are not always active.

By clearing a pilot to fly “Direct to XXX”, the controller shortens the route, saves fuel, and makes the pilot’s life easier, and this can also solve conflict problems over crowded intersectons. With modern navigation devices like GPS, but also inertial systems, it is possible to fly a direct to nearly every point on the globe.

A controller told me the story of a British Airways Boeing 747, shortly after inertial systems were introduced, who requested direct navigation to Cape Town… from Biggin-Hill VOR, in the London area. There was not a log of traffic, so he got it !

If you liked this post, have a look at the three things I don’t like to hear from ATC.

“Negative”
That one is rather clear: the controller can not accept my request. It is usually followed by cause, which can range from traffic to reserved airspace, via many others. It’s never pleasant, but pilots know, understand, and accept that. Depending what is denied, the plans have to be slightly or drastically changed, or even cancelled.

“Radar Service Terminated”
There are many ways to keep IFR traffic separated from each other. The most obvious and frequent is use of radar by controllers. They know the position of each aircraft, and provide guidance. It’s convenient and efficient. It’s somehow reassuring to know that I’m a “blip” with a position, a label, and an altitude on the controller’s screen, even if it does not take any of my pilot’s responsibilities away.

But radar coverage is not present everywhere, particularly in lower airspace and regional airports. Aircraft are then kept separated from the other and from the obstacles using procedures. Typically, all aircraft are kept in holding at different altitudes, and fly the approach one after the other. It’s perfectly safe, as long as pilots flow procedures as published. In case of deviation however, there won’t be any friendly voice in the radio to issue a warning.

When leaving radar coverage, or before transfer to a non-radar equipped ATC service, comes that famous “radar service terminated” message. It always sounded like a “don’t mess it up now, you’re on your own, kiddo”.

“Say intentions”
From all the standardized vocabulary used between controllers and pilots, this is probably the worst sentence of all. Controllers use it when they have no idea what a pilot will do, either because something got denied (see “Negative”) and there are no obvious options, or because the pilot just declared an emergency.

Typically, if a pilot has doubts regarding the landing gear extension, he will require a low pass in front of tower for visual inspection. The controllers will look using binoculars, and then report to the pilot. This could be something like “Sorry, no landing gear visible. Say intentions”.

For a pilot, not having a plan is not a glorious situation. In the best case, “say intentions” means that I’m with no plan, and in the worst case, it means that I’m in an emergency. There is only one positive thing in “say intentions”: ATC will help to solve the problem, whatever it is.

If there are other things you don’t like to hear, or if you want to take part in the preparation of the “Three things ATC don’t like to hear from pilots” post, leave a comment.

Air traffic controllers are able to keep planes separated using only radio and procedures. Typically, all aircraft flying to a particular airport will wait in a holding pattern, and one will be authorized to start the approach every 3 minutes. This is not optimal, but without a detailed knowledge of the situation, it’s the only possible way.

Radar is used to gain this extra information, making higher capacity possible. The first kind of radar, known as Primary, is based on waves reflection. The radar sends a pulse in a certain direction, and measure the time until it receives the echo. Knowing the propagation speed, it’s easy to determine the distance of the target. As the wave is sent in a narrow beam, the azimuth is well known.

As no special equipment is required on board the plane, this is technique is known as non-cooperative. On the other hand, it do not detect only aircraft, but all what can reflect a wave… Other limitations are the absence of altitude detection, and the large power required, as the wave has to travel back and forth. Primary radars use typical parabolic-ish antennas which make easy to identify them.

The second antenna, looking like a grid on top of the primary antenna is for secondary radar.

The Secondary radar is not based on reflection, but relies on a response message sent by an on-board equipment, the transponder. Each time the transponder is interrogated by a radar, it sends a code the pilot did set, and possibly the aircraft altitude. This makes possible for controllers to match echoes with planes, which was not possible with primary radar, except by requiring aircraft to fly some manoueuvers.

As all transponders in the radar beam send an answer, it’s not always possible for the receiver to identify them, leading to so-called garbling situation. This is particularly true in holding stacks. Another limitation is the very limited number of possible codes - only 4096. Obviously, this kind of radar works only if the transponder is answering, and it’s possible to cheat by delaying the answer.

The new generation of secondary radar - known as Mode-S, standing for selective - uses a new kind of transponder. The code is still present, but each transponder also has a specific address, making possible for the radar to interrogate only one transponder in the beam. This reduces garbling problems, and provide some help to solve the code shortage problem. Mode-S is also able to send more information, like the aircraft tail-number or commercial call-sign, and data computed on-board as well as Flight Management System settings.

The radar image made available to controllers normally relies on several radars. All the echoes are processed by a tracker, resulting in a better quality, smooth, and accurate picture. With appropriate radar coverage, it is legal to have aircraft flying with 2.5 to 5.0 miles between each other. Detection technology could allow for much shorter separations, but this distances are also needed to allow for some reaction time in case of problem.

There are other kind of radars used in ATC, like ground radars, and precision approach radars (PARs). PARs are mainly used on military airports, to literally “talk” airplanes down. This is unfortunately usually not accessible to civilian aircrafts, except for training purposes… mostly in the UK.

Radar technology has been around for a while now, and new solutions start to be used, including Automatic Dependent Surveillance (ADS) and Wide Area Multi-lateration (WAM), and I’ll soon post about them. If you have any question about surveillance technology, feel free, I have some knowledge about that…