I recently spoke to a controller who looks after an area just to the north of London. This area is very popular with light aircraft. She tells me that she quite often handles aircraft which are equipped with a working transponder, but which are not providing any altitude information. She tells me that when she asks them to "Sqwark altitude if available" that 8 out of 10 of them (yes - 80%) come up with a correct altitude read out within a few seconds.
WHY IS THIS SO?
There are several reasons why an aircraft which is fitted with an altitude encoding transponder might not be providing altitude information.
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(1) The altitude encoding equipment is broken altogether. The crew may or may not be aware of this, but can always ask ATC.
(2) The altitude encoding equipment is working but is providing incorrect altitude information, and the crew has been asked to turn off the altitude information by ATC.
(3) The crew has switched the transponder to "ON" rather than "ALT" position, thinking that "ON" is the correct setting, and possibly not understanding what the "ALT" position is for.
(4) The crew has switched the transponder to "ON" in the knowledge that this will not provide altitude information because they don’t want to get caught for being in the wrong place at the wrong altitude - just in case their navigation and altitude keeping is not as good as it might be. ie an inadvertent penetration of controlled airspace.
(5) The crew has switched the transponder to "ON" in the knowledge that this will not provide altitude information because they know that they are or soon will be in the wrong place at the wrong altitude. ie a deliberate penetration of controlled airspace.
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Situations (1) and (2) can and do happen, although rarely.
Situation (3) is a pilot education issue, and I would ask all those who are instructors, or who might be involved with aero clubs, or who might otherwise have contact with pilots in general to spread the word that transponders need to be switched to "ALT".
On transponders which do not have an altitude encoder fitted, the “ALT” position will work exactly the same as the “ON” position. A pilot who is unsure whether the aircraft has an altitude encoder or not should be aware that you can not do any harm by selecting “ALT” even if an encoder is not fitted.
(4) is understandable but unwise, while (5) is inexcusable.
Please, everyone, switch your transponders to “ALT”.