Herbert Degan III

33 Yrs FAA ATC Tower / Radar Experience

FAA Licensed Private Pilot / Commercial Rating

1800+ Total flight hours

FAA UAS Part 107 Remote Pilot

Essential Points Regarding ADS-B and Altitude:

Pressure Altitude: The altitude reported by ADS-B is recognized as "pressure altitude." This measurement is influenced by the prevailing atmospheric pressure and the altimeter setting utilized by the aircraft.

Altimeter Setting Source: The aircraft’s transponder employs the same altimeter setting source to determine and relay the ADS-B altitude data. Altimeter pressure settings are manually adjusted by the pilot. Look at the knob on the lower left corner of the altimeter graphic above to the right. That know will move the needles on the altimeter based on the pressure reading which are located between the number 2 and 3 on the altimeter dial. 

Importance of Correct Altimeter Setting: To ensure precise relative altitude readings between different aircraft, it is imperative that all pilots operating within the same vicinity adhere to the correct altimeter setting. Aircraft using the same pressure setting will display close to accurate altitude data. (Within tolerance) 

Air Traffic Controller Requirement: Air traffic controllers are mandated to provide the local altimeter setting to all aircraft within their jurisdiction. Pilots must then adjust their aircraft altimeters accordingly to ensure that all reported altitudes are accurate and consistent.

**Interesting Fact:**

Altimeters utilize variations in barometric pressure to indicate accurate altitude. It is essential for aircraft flying in close proximity to have the same barometric pressure setting in their altimeters. Barometric pressure constantly changes and can vary at different locations and altitudes. As aircraft navigate through airspace with different pressure settings, pilots must regularly check and adjust the barometric pressure on their altimeters.

Both altimeters at air traffic facilities and those in aircraft have allowable error tolerances. In practice, two aircraft that believe they are a few hundred feet apart might, in reality, be at the same exact altitude. For example, consider Aircraft A (Acft A) indicating 1000 feet on their onboard altimeter, calibrated for a pressure setting of 30.01. Another aircraft, Aircraft B (Acft B), indicating 800 feet on their onboard altimeter, also calibrated for a pressure setting of 30.01, could potentially be at the same actual altitude.

An error margin of ±75 feet is common. Therefore, if Acft B indicates 800 feet but might actually be at 875 feet, and Acft A indicates 1000 feet but might actually be at 925 feet, the space between the two aircraft narrows to only 50 feet. This scenario emphasizes the importance of ensuring that both aircraft have the same pressure altitude setting manually dialed into their altimeters by each pilot.

Consider another example: if Acft A is indicating 1000 feet in the cockpit and transmitting this to ATC but does not have the correct pressure altitude setting in their altimeter instrument, for every inch of mercury off, the altimeter will read 1000 feet off. If the pilot turns the knob on the altimeter instrument from 30.01 to 31.01, the altimeter would read 0 feet. Conversely, if the knob is turned to 29.01, the altimeter would read 2000 feet. Both of these altitudes would be transmitted to ATC as the actual altitude above sea level. During flight, if the pilot adjusts the altimeter knob, the indicated altitude will change on their altimeter and the Air Traffic Radar Scope. Controllers only see the altitude transmitted by the aircraft, whether accurate or not.

Controllers are trained to recognize discrepancies between the altitude reported by the pilot and the altitude indicated on the scope. When a pilot contacts a controller, they are required to provide their altitude. If there is a significant difference between the reported altitude and the indicated altitude, the controller will ask the pilot to confirm their altitude. Sometimes, the controller may instruct the pilot to stop using faulty equipment and instead advise when they are changing altitudes.

As illustrated with Acft A and Acft B, even a small error in one altimeter pressure setting can cause a serious problem. Altimeter calibration and certification may result in two aircraft, believed to be 200 feet apart, being only 50 feet apart (one at 875 feet and one at 925 feet). Controllers must regularly inform pilots of current pressure settings, as they constantly change. Controllers strive for all aircraft in their airspace to use the same pressure settings to provide ATC with the most accurate altitude information. It is common for aircraft traveling across the country to frequently check and adjust their pressure settings as the pressure changes.

FLYING INFORMATION