How High is High?

By Bob Turner

MCFI San Diego

I remember when GPS just got going good.  A buddy had the magic Garmin 295 and showed me how wonderful it was – it even had accurate altitude.  He could now be assured of being at the correct altitude no matter what.

I tactfully offered that maybe it was better to be at the altitude that everybody else was expecting him to be, and maybe that was different from the honest-to-gosh correct altitude measured by the GPS system.  Indeed, the way we select cruising altitudes in airplanes is by using matching instruments that may or may not know the actual altitude at which they are currently operating.  We get away with that because all airplanes in the vicinity are using equally inaccurate altimeters.

When you are cruising IFR and the center gives you an altimeter setting, it is probably not a perfect setting for where you are.  In fact, it is almost guaranteed to be accurate only at one point in the sector, maybe right outside the radar room.  You get the picture – everybody is using the same semi-accurate setting, regardless of the actual pressure at any given location, and separation is maintained – except for the occasional person who cruises around with a GPS altitude.

I think I gently convinced him to go back to the altimeter for his cruising altitudes.  

Today one of my students complained that his aircraft had a bad encoder.  It had a bad encoder two years ago, too, and we had it adjusted.  We had the transponder guy bump the reported altitude up a bit.  Turns out that the student had been cruising at the altitude on the face of the transponder, and not according to the altimeter.  The transponder is one of those fancy Garmin units with liquid crystal displays, and it reads pressure altitude right there on the face. 

And of course, since the controllers were still squawking after we re-set it, he assumed the transponder was sending a signal to the ground that was not matching the readout on the transponder.

The transponder was probably working just fine, except for the possible bump we put in two years ago. The transponder/encoder setup is designed to transmit the aircraft’s pressure altitude. In fact, the radar controller’s equipment is designed to apply a correction to the raw data from responding aircraft, commensurate with the altimeter setting the controller has selected for a given sector.

Pressure altitude, loosely speaking, is the altitude your altimeter reads when you set it to 2992.  If your reporting station is calling the altimeter setting 2992, you can be pretty sure the altitude your encoder is sending out is in fact the altitude shown on your altimeter.  But 2992 is a rare altimeter setting – it is usually a good bit higher or lower than that.

For instance, an altimeter setting of, say, 3012 will have the altimeter off by 200 feet from the altitude reported by the encoder, and that is enough, these days, to get a controller quite upset with you.  That is, if you cruise at 4,500 feet on the transponder readout, and the controller says the altimeter is 3012, assuming your equipment is working perfectly you will be 200 feet off your assigned altitude on the radar scope, and you will get fussed at (that’s how my Texas buddies phrase it).  

As an example, today the altimeter setting is 3024 - not unusual for sunny Southern California.  If I put that in my Kollsman window I read 420 feet – field elevation.  If I then put 2992 in the window, I get airport pressure altitude, which is 140 feet.  The 280-foot difference is enough to start alarm bells at a radar facility. Add the 100 foot error we erroneously introduced in my friend’s encoder, and there is the possibility of a violation under IFR or a class B transit.

Here is how you do it:  get your transponder/encoder checked every two years, which costs around $75, and it is required by FAR.  Then, use your altimeter for cruising altitudes, with the setting given you by the last controller.  Ignore your GPS altitude, and above all, ignore the altitude your transponder is giving you.  If you use that little number on the transponder, and you are on an IFR clearance, you could be impacting safety and you surely will be getting some heat from the radar specialist.

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