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Ep 36 – Erroneous Air Data

Podcast | May 13, 2024

In The News: Approach Minima Change in Canada


This podcast I’m starting with the in the news segment as I find this bit of news fascinating but not surprising. Our devices are under constant attack from malware, phishing scams so why not aircraft? The term being used is ‘aircraft spoofing’ and there have been a few occurrences in different parts of the world.


The FAA issued a warning recently…


Opsgroup “the FAA released a memo for aircraft operators titled “Iraq/Azerbaijan – GPS Jamming and Spoofing Poses Safety Risk“.”

The memo advised that “Potential spoofing activities reported by various civil air operators in Iraq and Azerbaijan pose a safety of flight risk to civil aviation operations in the Baghdad (ORBB) and Baku (UBBA) Flight Information Regions (FIR).”

“The recent opensource reporting regarding spoofing incidents, if confirmed, would pose increased safety of flight risks, due to potential loss of aircraft situational awareness and increased pilot and regional air traffic control (ATC) workload issues, which can lead to potential accidents and/or loss of life.”

“The FAA recommends that U.S. civil air operators transiting ORBB and UBBA monitor regional NOTAMs, put additional emphasis on maintaining continuous communications with appropriate air traffic control authorities while monitoring aircraft equipment performance closely for any discrepancies or anomalies, and to be prepared to operate without GPS navigational systems.”


Refer to your aircraft procedures for GPS failure. I’ll be talking more about GPS spoofing in a future podcast.



Air Data Failures


Today’s podcast I want to talk about air data failures. Consider the ramifications of an air data failure. There could be many alerts and distractions as a result of the failure or there could be very few. Does your aircraft have an air data mis-compare annunciation or a failure checklist such as ‘altimeter disagree’?


Incorrect altitude information from a faulty air data computer in a Cessna Citation resulted in a near-midair collision with an Embraer 170. Because of the Citation’s erroneous data, the Embraer’s TCAS failed to warn of the close call, while the air traffic controller believed that the altitude separation was adequate based on radar data.

On climb out the pilot of the Citation observed that the left and right altimeters were giving different indications, and the crew didn’t inform ATC of the discrepancy until they were en route. Controllers informed the Citation crew of converging traffic two miles ahead and 1,000 feet higher than the business jet. In reality, the Embraer was at a distance of 1.5 miles and 665 feet lower than the Citation. No ACAS/TCAS warning was given to either airplane or the controller because the systems had analyzed erroneous data from the Citation.


The investigation found that the fault on the Citation’s left side altimeter and airspeed indicator had occurred three other times on this airplane—in 2017, 2019, and 2021. Investigators believe that all four events could be traced to a low point in the pitot system where water or other contaminants caused an obstruction.


Contributing factors listed in the final report included:

  • the crew’s failure to notify ATC immediately of the altitude anomaly,
  • the controller not giving clear information to the crew concerning the altitude information on his screen in response to the crew’s questions, and
  • the absence of a manufacturer or operator crew procedure to deal with cases of air data faults.


Incident 1

On 14 August 2020, the pilot of a Cessna CJ2 encountered conflicting air data during the take-off from an airport near Paris. He observed inconsistencies in altitude and airspeed information given by each PFD. The pilot did not comply with the aircraft procedure dedicated to inconsistent altitude and airspeed information on the PFD instead he tried to clarify the situation with help from the controller which resulted in the pilot and controller deciding that the correct altitude was coming from the transponder. In fact, the transponder was providing the incorrect information all along.


The controller asked the pilot who was flying in cloud, to descend to 1,500 ft. The altitude displayed on the radar screen was close to 3,000 ft whereas the aircraft was actually flying at an altitude of approximately 1,300 ft. An EGPWS warning was activated onboard the airplane, and the recovery was successful. A dead bug and sand were found in a static port.


Incident 2


And finally, the craziest occurrence I found happened…


On 2 June 2010, when a near-collision occurred between an Airbus A318 operated by Air France and a Pilatus PC 12. The 318 was at FL 290 and the PC 12, cleared for FL 270, was actually at FL 290 due to an altimeter problem. The Airbus crew made an emergency evasive maneuver to avoid a collision. The minimum separation between the two airplanes was so close it could not be measured on the radar recording. It was estimated by the crews as being between 50 and 100 feet horizontally and around 100 ft vertically. There were no TCAS alerts.


The French bureau of investigation found..


“a leak of the static pressure line supplying the left side barometric and speed unit. This leak caused erroneous altitude and speed information. “


Like I said there was no TCAS alerts for either airplane or was there an alert on the ATC radar screen. There was an airspeed difference between the left and right sides but further investigation by the crew was not carried out. ATC and the crew believed the altitudes they were seeing and left it at that.


The flight level displayed on the ATC ground systems did not make it possible to dispel the doubt and thus led all of those involved (crew and controllers) to believe a flight level for the airplane that was erroneous. Due to this, the crew did not search any further for the causes of the inconsistency in the speed observed on the left side unit.” This particular aircraft system provided erroneous altitude data to ATC but only showed erroneous airspeed data to the crew.


Consequently, the French bureau of investigation recommended to EASA “that these cases be considered as emergency situations that must be declared without delay by crews to ATC services.”


Checklist for altitude disagreement


The airplane I fly has a detailed checklist for altitude disagreement between instruments. Firstly, the checklist directs you to go the airspeed disagree checklist first if airspeeds are different, then to check that altimeter subscales are set correctly on each altimeter. Then check the standby altimeter and find clear visibility conditions if possible. Then the checklist asks if you can determine if you have a reliable altimeter. With 3 altimeters on board if two are the same then the other should be disregarded. If you can do this then you should make the transponder read from that side PFD, so left side altimeter and standby agree so set the transponder to number 1. If you cannot determine which altimeter is correct, then set the transponder to Altitude off.


Other things to consider before the checklist is complete are to only use baro-minimums functionality on the functioning side for approach, refer to your radar altimeter display on approach to double check your altitude and use ILS or vertical navigation for the approach. ATC can be helpful but they don’t know your aircraft systems like you do.


At my operation whenever an abnormal or emergency situation occurs the flying pilot says, ‘what’s the problem?’ Pilot monitoring says, ‘altimeter disagree’. Pilot flying says ‘altimeter disagree non-normal checklist. I have ATC’ The flying pilot could notify ATC they are running a checklist and any requests with regards to flight profile or wait until the checklist is complete.


Thanks for listening and have a great day.





Citation’s Erroneous Altitude Data Led to Near-collision


FAA Warning Issued, Further Serious Navigation Failures Reported


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