Aviation In Canada

Saturday, May 14, 2005

Mode C Altitudes

A common misconception is that the altitude reported by a Mode C transponder is the altitude displayed on the altimeter of the aircraft. This is not the case. The Mode C transponder always reports altitude based on 29.92, regardless of what's in the calibration window on the aircraft's instrument. This means the Mode C reports pressure level, not altitude. So if a pilot is flying at 3,000 feet on an altimeter setting of 30.42, he is flying at 3,000 feet, but his mode C is actually reporting an altitude of 2,500. Effectively, the Mode C's altimeter is set a half inch to low, so it will read 500 feet lower than the aircraft actually is.

ATC's radar now takes over. For each area, an appripriate altimeter is fed into the processors to correct Mode C altitudes. For example, over CYQM, the altimeter for the Moncton airport is fed in and used when correcting Mode C readouts on aircraft flying in the Moncton area. Our example aircraft above reports the altitude of 2,500 feet to the radar. The radar processors consider the altimeter difference between standard pressue and station pressure, and correct the Mode C readout to show 3,000 feet.

This process, as you can figure, may lead to some errors. For example, if the actual sea level pressure where you're flying is higher or lower than what the radar processor is using to correct your Mode C, it'll reflect an errant altitude. Just how much depends on the difference, but it is usually not much. In any case, ATC may have given you the same altimeter setting since you're in the same area, so your altitude and Mode C should jive nicely.

The nice thing about it is that it gives a cross check. Redundancy is what makes the Canadian Air Navigation System as safe as it is. Now, this isn't exaclty a Canadian idea, since this is the way things work worldwide, but it is a good idea nonetheless. For example, it helped us catch an error in the weather sequence one evening, that could have lead to serious problems. An aircraft was climbing out of Moncton westbound, cleared up to 12,000 feet. On initial call to the next sector, he was given the local altimeter setting. A few minutes later, he reported level at 12,000. His Mode C readout on the radar showed 11,000. The standard practice is to issue the current altimeter setting and ask the pilot to verify is altitude. The pilot confirmed he was showing 12,000 feet, despite our reading of 11,000. This is something that warrants further investigation, of course. Looking at surrounding weather sequences, it was quickly determined that the altimeter setting he was issued was vastly different from other stations in the area. A call to the FSS involved revealed that a trainee had inadvertently reported the altimeter off by 1 inch, giving our radar processor an invalid altimeter setting to use for correcting Mode C altitudes. Shortly thereafter, a correction was issued (and maybe a good cuff in the back of the head for a trainee) and everything was right in the world again.

Also, this can help ATC detect when a pilot has the incorrect altimeter setting in the window. If a pilot sets the altimeter incorrectly, he'll report reaching an altitude, and this won't show the same as what ATC sees. Then, steps can be taken to ensure the pilot has the correct altimeter. Critical especially when an aircraft is on approach in IMC. If the Mode C actually did report what the pilot was seeing on his altimeter, this level of redundancy would disappear, and ATC would have no chance to catch an error of this type.

It seems a little convoluted until you think of how it works, and suddenly, it seems like a good thing, doesn't it?