In aviation, even small mistakes can have serious consequences, and one such error involves setting the wrong QNH.
For pilots, a mistake in setting QNH—referred to as a QNH blunder—can lead to altitude deviations, compromised terrain clearance, and, in extreme cases, accidents.
What is QNH related to?
Pressure at aerodrome level
Pressure oustide the aircraft
Pressure at the Pilot Theory Online HQ
Pressure at mean sea level
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Pressure at mean sea level
What Is QNH, and Why Does It Matter?
QNH is the atmospheric pressure at mean sea level (MSL), calibrated to give pilots an accurate altitude reading above sea level. It is a critical part of pre-flight and in-flight operations, particularly during takeoff, approach, and landing. If QNH is set incorrectly, the altimeter will display the wrong altitude.
If we set too low a QNH: The altimeter reads too high, or we are lower than the altimeter is saying (risk of terrain or obstacle impact).
If we set too high a QNH: The altimeter reads too low, or we are higher than the altimeter is saying.
While QNH errors may seem minor, in busy or mountainous areas, the consequences can be catastrophic.
A Real-Life QNH Blunder Incident
One notable incident occurred in Birmingham, UK, in 2009 involving a Boeing 737 during its approach.
What Happened?
The crew received the incorrect QNH (pressure setting) of 976 hPa instead of the correct value of 996 hPa—a 20 hPa difference.
With the wrong QNH, the altimeter indicated an altitude 600 feet higher than the aircraft’s actual altitude.
During the approach, the aircraft descended dangerously close to the terrain while the crew believed they were well above the minimum safe altitude.
How It Was Discovered
Air Traffic Control (ATC) alerted the crew about their unusually low altitude based on radar.
The crew quickly corrected the QNH, recovering safely.
Key Takeaway
A simple input error caused the altimeter to mislead the crew. If not for ATC intervention, the aircraft could have collided with terrain.
This incident highlights how a minor oversight can escalate into a hazardous situation, especially during critical phases of flight like approach or landing.
Common Causes of QNH Errors
Miscommunication: Mishearing or misunderstanding ATC instructions for QNH (e.g., "nine nine six" getting confused with "nine six six").
Human Factors: Fatigue, distraction, or workload during busy phases of flight can lead to input errors.
Transition Errors: Forgetting to adjust from local QNH to standard pressure (1013 hPa) when passing the transition altitude or vice versa.
Regional Differences: Variation in units—some regions use hectopascals (hPa), while others use inches of mercury (inHg), creating confusion for international pilots.
Confirmation bias: It's common for pilots to have a pre-set expectation of what the QNH should be. If this expectation is actually incorrect, they might still go with it because they think it is correct.
Techniques to Prevent QNH Blunders
Preventing QNH errors requires vigilance, good communication, and adherence to best practices. Here are techniques to minimise the risk:
1. Use Standard Readback Procedures
Always read back the QNH received from ATC, word for word, to confirm accuracy.
Example: “QNH 996 hPa, Boeing 737 confirming nine nine six.”
This allows ATC to catch any discrepancies or misunderstandings.
2. Cross-Check Altimeter Settings
During pre-flight and descent preparation, both pilots should cross-check QNH settings.
Compare your altimeter readings with known airport elevation before takeoff or approach.
If the altimeter reading doesn’t match, double-check the QNH.
3. Utilize Automation
Many modern aircraft allow QNH settings to be entered into the Flight Management System (FMS), providing a visual cue for verification.
Pilots can also use approach plates and onboard weather systems (e.g., ATIS or METAR) to confirm QNH values.
4. Be Extra Vigilant in Transition Altitudes
Know the transition altitude for the region you are operating in.
Double-check that you switch between local QNH and standard pressure (1013 hPa) at the appropriate altitude.
5. Reduce Fatigue and Workload
High workload and fatigue can cause errors to slip through. Brief altimeter changes and confirm QNH well ahead of time during approach preparations.
6. Practice CRM (Crew Resource Management)
Effective communication between pilots is essential.
The non-flying pilot (monitoring pilot) should always verify inputs made by the flying pilot, including QNH.
7. Do a precision approach where possible
Here we mean an ILS. The ILS glideslope beam comes from the ground and couldn't care less about the QNH.
As soon as the aircraft is locked onto the glideslope, QNH blunder error isn't really an issue anymore.
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