Trivia Testers: Altitude Encoding

FAA regulations require that you have a Mode C transponder on when flying in all Class A, B and C airspace, when flying within 30 miles of primary Class B airports, and whenever you’re above 10,000 feet, though not if that’s below 2,500 AGL. (And there may be more, as a result of the events on September 11.) As you probably know, there are two ways ATC radar can see you. There’s the ‘primary‘ echo, used since WWII, of microwave energy bounced back from a ‘skin paint.’ It divulges only azimuth and range (though not too well if you’re flying a small composite airplane, are far away, or there’s precipitation in the way). Then, there’s ‘secondary‘ radar, which works when you’re further away — up to about 100 miles.

Part of it is that little box in your panel with the four octal numbers called a transponder (a contraction of ‘transmitter-responder’) with that four position switch labeled OFF, STBY, ON and ALT. (Some have a fifth, called TEST.) They also have an IDENT button, a reply light, and four numbered wheels (except for the newer ones with a digital presentation). This device answers a separate ATC antenna with whatever four digit code you’ve dialed in (known as Mode A) as well as your altitude, if you’re using Mode C capability (i.e., you’ve switched it to ‘ALT’). Aha! But what altitude? And just where does it come from?

There are basically two types of altitude encoders.

  1. The cheapest kind is the blind encoder, generally a solid state affair. It’s a separate box, costing between $175 and $400, whose sole purpose in life is to feed the transponder a digitized altitude, which it gets from a separate static pressure transducer.
  2. The second kind is also a ‘blind’ encoder, but it’s built INTO the altimeter, which does save a little space. (This one costs between $800 and $1800 to overhaul!)

Similarities: BOTH are calibrated at a pressure altitude datum (reference) of 29.92 and relay (or transmit) altitude — to the nearest 100 feet — corrected to 29.92. ATC’s ground equipment makes the altitude corrections directly onto the controller’s screen. What you dial up in the Kollsman window on your altimeter has absolutely no effect on what they see on their screens. Skeptical? Even Jeppesen’s Private Pilot Manual says that an incorrect altimeter setting has no effect on Mode C readouts, since the equipment is ‘preset‘ to 29.92.

Some little-known facts about transponders…

  • COST: The cheaper solid-state transducers become inaccurate with age, meaning that you might wind up spending more in the long run so your avionics shop can keep it ‘in spec’. The second type (aneroid type) encoders are costlier, but they seldom need adjustment.
  • TESTING: FAR part 91.217 requires the data correspondence between the altimeter and the encoder to be biennially tested and calibrated to transmit altitude data to within 125 feet of the indicated or calibrated datum of the altimeter used to maintain flight altitude.
  • POWER: 200 watts is typical. So if you’re alternator ever goes and you need to conserve power in an emergency, you might consider taking it offline — if ATC can pick you up on primary radar.
  • PROCEDURES: After you start your engine, turn your transponder to STBY. It’ll warm up, but won’t respond to interrogations. As you proceed to the runway, set it to the ALT position, because even if you don’t have an altitude encoder, the ground unit is still looking for Mode C ‘framing pulses‘ from your transponder, even if there’s no altitude information accompanying them.
  • MISTAKE: If you take off IMC and then realize you forgot to turn it on, you might put yourself in trouble because some can take several minutes to warm-up before they’ll answer interrogations.
  • ACCURACY: A recent FAA field study done by the Hughes Technical Center in Atlantic City, in which about 30 separate parameters on over 500 transponders were tested, showed that only 4% were able to meet all the specs. Also, about one-sixth of the tested transponders would create significant problems when interacting with ground Secondary Surveillance Radar processors, or TCAS. In another study, the FAA found that more than 20% of the transponders it checked failed to report ANY altitude at all, and about another 4% had large altitude errors.
    Moral: Pilots who use the ATC system get instant feedback on whether their transponders are working properly. Pilots who don’t fly IFR or ask for VFR flight following may never be aware of a malfunctioning transponder.
  • BLOOM: Whenever a controller asks you to ‘squawk ident‘ or ‘ident,’ you simply push the button on the transponder marked IDENT, which causes your target on the controller’s radar scope to either ‘bloom‘ on an approach control radar screen, or has a flashing ‘ID‘ on a center radar screen. Controllers sometimes use this to help find you (on their screens, that is).
  • OPERATION: The only time you should ever operate your transponder in the ON (rather than ALT) mode is when ATC specifically instructs you to do so… and it probably means there’s a discrepancy in your altitude read-out.
  • OTHER: There’s another mode called Mode ‘S’ (for ‘select‘), required for TCAS and maybe, someday, datalink. Aircraft with Mode S transponders each have their own unique 24-bit address.

And if you’re really into trivia: Did you know there are actually over a dozen types of ‘altitude‘ indicators on a controller’s data block? Yup. According to FAA Technical Center spec 314 (called something like the ‘NAS Enroute Configuration Management Document‘), it’s ‘Field B-4‘ and it’s called the ‘Assigned Altitude and Altitude Qualifier‘. (There’s also a Field ‘C‘, which is a reported altitude or assigned block or Mode C ‘disestablishment indicator‘.)

The B4 characters are:

A = reported altitude equals assigned altitude
N = no beacon reported or controller-entered altitude, or a/c’s rate of change is questionable Up Arrow = altitude below assigned while climbing Down Arrow = altitude above assigned while descending
C = beacon reported altitude within conformance limits
+ = beacon altitude exceeds upper Conformance Limit
– = you guessed it; below CL
B = in conformance with assigned block
/ = flight is OTP or VFR
V = beacon reported or controller-entered when no assigned exists
F = reported altitude equals first altitude and within conformance limits
X = beacon reported altitude disestablished
T = interim altitude being displayed (B1 – B3)

We just thought you might like to know…