We know, we know. The CAA has spent the last several years encouraging you to get ADS-B. But we’ve also spent the last several years saying it should never be your only source of ‘lookout’.

Everybody knows the benefits of ADS-B. Or you should do, by now.

You should also know by now that ADS-B, despite its situational awareness game-changing qualities, has its limitations.

You can never know, for instance, which of the aircraft sharing your airspace actually has ADS-B installed.

You can never know whether the ADS-B tech they might have installed is actually switched on or functioning properly.

In Ketchikan, Canada, in 2019, two aircraft collided after the ADS-B in one aircraft was not broadcasting pressure altitude, which it needed to do to provide an alert to the kit in the second aircraft. Five died and nine were seriously injured.

ADS-B in your aircraft, therefore, does not guarantee collision-free airspace. In the Vector article I fly outside controlled airspace (Spring 2021) we reported that a near miss occurred in the Bay of Plenty between a fully equipped (IN and OUT1) aircraft and a non-equipped aircraft.

“The ADS-B pilot heard no radio call (it was in an area where a frequency change was due) and, of course, no OUT transmission showed up on the IN display.

“Fortunately, through sheer luck, the aircraft avoided each other. But it could have had a catastrophically different outcome.”

In the Vector article Advice from ADS-B equipped pilots (Winter 2020), North Shore pilot Steven Perreau said of his new kit, “You can never assume that a lack of traffic on the display means there’s actually no traffic around. You’d be a mug to use it 100 percent instead of the mark 1 eyeball”.

Pilots also need to be aware of the limitations of ADS-B IN equipment that connects to the device running your favourite electronic flight bag app.

With portable ADS-B IN receivers that sit in your cockpit, a line of sight to the aircraft transmitting the OUT data is required. An obstruction such as the aircraft body itself can block these transmissions from being received.

Otago Aero Club CFI Joe Calder says it’s also important to make sure the device is up-to-date because, if it’s not, the ADS-B IN can work incorrectly.

“ADS-B IN is a great tool, but there’ll always be some non-ADS-B aircraft flying. Keeping those eyes outside the window, maintaining a thorough lookout, is most important.”

You also need to be aware the display device you’re using for your ADS-B IN receiver may be showing incorrect altitude readings.

ADS-B OUT data is set to a standard pressure setting of 1013.25 hPa.

This could potentially mean that an aircraft registering 100ft below, could be 100ft above.

Either way, treat altitude information on visual displays with caution.

You’re also not necessarily going to see the aircraft detected on your display in exactly the location expected when doing your visual lookout, as there can be some lag.

As South Island pilot Ian Sinclair said in the previously mentioned Vector article, “Even though it has quite good eyesight, ADS-B IN is still only one tool in the awareness shed.

“Lookout, good radio work, and predictable flight patterns all need to be maintained.”


ADS-B OUT allows an aircraft to broadcast its position to other aircraft (who have ADS-B IN) and to ground receivers (like Airways). ADS-B IN allows an aircraft to receive those broadcasts.

Posted in Airspace, Pilot performance flying practice and professionalism;

Posted 5 months ago