Expert: Marc Ebelini - 5/28/2010

Question
QUESTION: If your aircraft has only a blind encoder and not an encoding altimeter, how does ATC center compensate for the altitude info it receives which is based on 29.92 setting and not corrected for the current real barometric conditions? Thank you in advance. Paul

ANSWER: All encoding data from the altimeter (a pressure reading device ) or encoder is corrected to the local barometric pressure in that operating area automatically. Must remember an altimeter is not a very accurate altitude source but a very accurate pressure measuring device. The altitude is merely computed based on the pressure reading. The 29.92 is used as the reference point. For a given pressure using 29.92 as the reference pressure and 59 F as the temperature then the pressure altitude scale was created. Since the density of the air varies with temperature the actual altitude will vary accordingly. So never rely on your altimeter giving you the actual height above ground. Beside how many people test their static ports other that the required leak test. Ports should never be painted or cleaned with a metal object. Waxing can also cause a problem if wax changes the airflow over the hole.
So dialing in that local barometric setting to is to put everyone at the same reference point to ground. Not necessarily the correct altitude above ground. Hope I was clear enough? To under stand it further get a sensitive altimeter and go so place where the ceiling is over ten feet. Place the altimeter on the floor and zero it. Lightly tap (instrument vibration) it then raise it ten feet or more and the altimeter should see the amount of height change. It did this by just measuring the difference in the air pressure from the floor to the ceiling. Shut off heat or AC air flow that may effect this test.

---------- FOLLOW-UP ----------

QUESTION: Are you saying that the altitude data that ATC receives from an encoding altimeter is based on the 29.92" setting and not on the manual setting set by the pilot? I understand that the altimeter indicates MSL or Mean Sea Level, and not AGL or Above Ground Level, I'm just wondering if ATC gets altitude data in the same format regardles of it being sent (through the transponder)from an encoding altimeter, or from a blind encoder. Or if there is some identifying information in the data that indicates the origin as being "Corrected" or "Standard Day" barrometric pressure referenced. Thanks again. Paul

Answer
As I explained before the altimeter or blind encoder are only pressure reading instruments. All they do is measure pressure. Both the altimeter and encoder transmits that pressure reading in the form of a gray code. What is so special about the gray code? Only one digit is changed for each hundred foot change in altitude. (they have 25 foot units too) Now as I explained, altitude is determined by the pressure reading. So as the aneroid expands the code wheel or other device used to measure pressure sends this pressure reading to the ground station. When you move your local barometer setting to the local local one you are in moving the hands on your altimeter away from what the encoder is sending to the ground. When an altimeter is calibrated the barometric setting is placed on 29.92 so that when lets us say we place the pressure for 500 feet the altimeter reads 500 feet, or 1000 feet it reads 1000.
Now lets try another thing. If an airplane on the ground tells you the Barometric pressure he sets his altimeter that reads zero will allow you to land at zero. Disregard any errors in the altimeters. But placing the local barometric pressure in your altimeter and landing the airplane you should read the altitude the airfield is at. Do you get the picture now?
This pressure amount for each altimeter test point is so accurate that when setting up a mercury barometer it requires that the the local gravity be known to correct for the effect of gravity on the column of mercury.
To repeat myself. both the altimeter and encoder sends the same pressure readings based on the altitude pressure scale. Does not matter where barometric setting is on your altimeter.