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How Bizav Pilots Can Mitigate Altitude Deviations at Non-US Airports

Non-U.S. airports sometimes introduce different risks for operators, resulting in pilot altitude deviations from ATC flight clearance – also known as level busts.

What makes some airports so challenging and how can pilots mitigate the risk of a dangerous level bust or other deviation?

Ireland’s Shannon International Airport (SNN) is a particular hot spot for level busts, considered to be a deviation of 300 feet or more from ATC clearance. Jonathan Byrne, operations manager at AirNav Ireland, said that North American-registered business jets account for 30% of the total approaches to SNN, and most of the level busts on approach.

This type of error is not limited to SNN; however, the common use of the airport as an entry point to Europe results in a high number of deviations occurring at that facility. That said, best practices offered by Byrne and other experts are relevant for airports around the world.

In the table below, 2020 and 2021 are excluded, as the global pandemic resulted in relatively fewer operations. It’s important to note that while the number of deviations in 2022 and 2023 were lower than 2019 and previous years, Byrne reported they are still significantly higher than the norm.

Level Busts on Approach at EINN

Year Total North American Business Jets U.S. Military Other
2018 18 12 4 2
2019 22 11 4 7
2022 11 7 3 1
2023 13 10 2 1
2024 (Jan.–Feb.) 1 1

Source: AirNav Ireland

Possible Causes Behind Level Busts

Byrne suggested the primary causes for these level busts are delayed change – or failure to change – from inches of mercury (hg) to hectopascals (hPa) and untimely change – or failure to change – to local QNH, which is the barometric altimeter setting that causes an altimeter to read airfield elevation above mean sea level when on the airfield.

A related cause is the significant difference in transition altitude, which is 18,000 feet in the U.S. but 5,000 feet at SNN.

Of the 24 occurrences in 2022 and 2023, AirNav Ireland confirmed 13 were due to incorrect altimeter settings. In three occurrences, the pilot reported autopilot issues. In two occurrences, the pilot reported deviations due to weather conditions, and the remaining six were self-reported “pilot error,” which were likely incorrect pressure settings.

Specific Level Bust Examples

Shawn Scott, co-founder of Scott IPC, shared an April 2019 scenario in which a business jet came within 2 nm and 500 feet of high terrain while on approach to Runway 24 at SNN. When cleared to 3,000 feet on QNH of 0988 hPa, the aircraft descended to 2,300 feet.

Scott’s team of expert analysts determined the likely causal factors to be failure to switch to local QNH of 0988 hPa, as standard QNH is 1013.2 hPa, a difference of 750 feet; or a entering a setting of 29.88 hg instead of 0988 hPa, a difference of 720 feet.

A similar event occurred in 2022, when a U.S.-registered business jet was cleared to 4,000 feet on a QNH of 0987 hPA on approach to Runway 24 at EINN. The flight crew incorrectly set the altimeter to 29.87 hg, a difference of 720 feet, so while the altimeter indicated 4,000 feet, the aircraft was at 3,300 feet.

Suggested Standard Operating Procedures

Experts agree implementation of effective standard operating procedures (SOP) are the key to mitigating the risk of errors on approach at non-U.S. airports.

These SOPs should include:

  • Changing to hPa prior to entering controlled airspace.
  • Use of challenge and response procedures between the pilot monitoring and pilot flying.
  • Reminders of the difference between U.S. and European transition levels.
  • Completing full readback, including unit of measurement (i.e., hPa), when issued QNH by ATC.
  • Verifying unit of measurement when issued a clearance by looking at the altimeter.
  • Notifying ATC immediately of any error or delay in proper altimeter settings.

Beware of ATC Hear-Back Errors

“To reduce the chance of ATC missing an incorrect read-back, if there is any misunderstanding, a question between crewmembers, or you’re not sure you heard the full clearance, don’t repeat what you think you heard,” Scott advised. “Ask ATC to repeat the full clearance and then read back the full clearance, including the unit of measurement for the altimeter setting.”

Scott said this will reduce the number of ATC hear-back errors. The crew may still make an error reading back, but by asking ATC to repeat the clearance, ATC will realize the clearance was not understood or well received in some way. This will naturally make ATC pay closer attention to your read-back. In some locations, ATC may ask the crew to not read back and just listen. Scott said this is unacceptable. ICAO requires that if the clearance is safety related, it must be read back.

Scott also suggested paying close attention to the individual country’s aeronautical information publication or airfield requirements as to setting QNH on the descent. In the case of the UK and Ireland, that procedure differs from the standard ICAO requirement of changing when descending through the transition level as the transition levels are so low and very close to the approach altitudes.

They want operators to change to QNH after receiving the clearance to an altitude for an approach. In one case an operator waited to switch until they arrived at the cleared altitude with QNE set. When they switched to QNH they were immediately 400 feet low and earned a level bust.

“Probably the No. 1 issue in Irish airspace and the UK is when to set the altimeter on descent because transition altitudes levels are very low,” said Scott.

Transition altitudes in the UK and Europe can be as low as 5,000 feet, and with the transition levels published to only provide a transition layer of 1,000 feet or so, that may be problematic if the crew is behind the situation.

Pilots can also use their aircraft Ground Proximity Warning System or Terrain Avoidance Warning System Class A with simple terrain mapping displayed to maintain better situational awareness or program the flight management system to the correct transponder setting.

SNN presents a bit of a perfect storm, with lower than typical transition altitudes, a change in altitude unit of measurement and even its place in the flight sequence: at the end of a long trans-Atlantic flight.

Fatigue Guidance and Software Upgrades

Pilot fatigue is also often associated with level busts. To mitigate the risk of a fatigue-related deviation, pilots and operators should work together to implement science-based fatigue risk management strategies, including limitations on duty time, required rest periods that account for time zone crossings and even strategic inflight use of caffeine.

In addition to pilots properly implementing SOPs, air traffic control is doing its part to mitigate the impact of a level bust. Byrne explained SNN will undergo a software upgrade in 2025, which will alert ATC that an aircraft has an incorrect pressure setting.

“This software upgrade will greatly enhance our ability to prevent level busts and more importantly [controlled flight into terrain] CFITs on approach. But it is important to state, this is a mitigation that we will implement to reduce the severity of the consequences of pilot error; it will not stop pilot error, it just gives us greater ability to protect the aircraft after the error,” said Byrne. “The true solution to the problem is to stop the error to begin with.”

“While Shannon is a hot spot for level busts on approach, pilots should be particularly on guard at airports with low transition altitudes. Ireland and the UK are unusual with low transition altitudes. We don’t see that a lot of places,” said Scott.

Overall, Byrne said, the “ultimate ability and responsibility to stop these errors from occurring in the first place remains with the flight crew, through crew education and development of functional SOPs specific to oceanic travel.”

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