QNH Errors on Approach: A Small Setting with Big Consequences
- Capt. Mark Walton FRAeS
- May 30
- 3 min read
Updated: Jun 1
Barometric pressure settings are one of the most basic inputs we handle on the flight deck - yet their significance is anything but basic. An incorrect QNH setting can quietly undermine even the most stable VNAV approach, potentially leading to altitude busts, terrain proximity, or worse.
This issue isn’t theoretical. As Airbus noted in their Safety First publication, real-world data shows that mis-set barometric pressure is a recurring causal factor in unstable approaches and Controlled Flight Into Terrain (CFIT) incidents. Let’s break down why this happens - and how to avoid it.
What Is QNH and Why It Matters
QNH refers to the local barometric pressure at sea level, used to calibrate altimeters to show height above mean sea level (AMSL). It’s crucial for situational awareness, terrain clearance, and compliance with approach procedures.
When we select VNAV for a non-precision approach - often an RNAV(GNSS) with baro-VNAV - the Flight Management System calculates the vertical path using QNH as a core input. An incorrect QNH directly corrupts the aircraft’s altitude reference, silently placing the aircraft above or below the intended profile.
The Risk in Numbers
As Airbus points out:
A 10 hPa error in QNH equates to an altitude error of about 280 feet.
At 500 feet AGL, a mis-set QNH of even 4–5 hPa could lead to premature descent below the Minimum Descent Altitude (MDA) or Decision Altitude (DA).
VNAV doesn’t “know” the QNH is wrong. It will still fly what it believes is the correct path.
Why VNAV Increases the Risk
VNAV approaches are designed for efficiency and stability - but unlike ILS, there’s no external guidance signal to compare against. Everything rides on the integrity of your barometric input.
If the QNH is off:
The vertical profile is corrupted from the start.
There are no onboard alerts.
The aircraft may descend below the final approach fix minimum altitude unnoticed.
As Airbus emphasizes, “barometric altitude is a derived, not direct measurement.” VNAV assumes QNH is correct - and so do pilots unless specifically challenged.
Best Practices from the Line and the Simulator
As a senior Airbus examiner, I’ve witnessed first hand how easily this can go wrong - especially in simulator checks replicating high-terrain environments.
Here’s what I recommend:
Set QNH early: Enter it during TOD setup — don’t wait for final descent.
Both pilots must verify: Cross-checking QNH should be as instinctive as checking flaps or gear.
Use radio altimeter as a gross error check: At 1000 feet AGL, check that RA roughly matches expected height above terrain. A discrepancy could indicate a mis-set baro.
Brief the risk: If flying into high-pressure change areas, include potential baro error as a threat in the Threat and Error Management briefing.
Final Thought: It's Not Just About Numbers
The QNH setting is a single line entry on a display - but it's also a line of defense. A few hectopascals off, and the safety margin you're counting on may vanish. Especially in VNAV approaches, this is one setting that requires old-school vigilance.
As Airbus aptly concludes: “Use the correct baro setting - and check it.” It’s simple advice. But it may be the one thing standing between a stabilized approach and a safety report.
Want to learn more about baro-VNAV, CFIT prevention, or training your team in advanced threat detection techniques?
Reach out to us at Yaw Aviation for customized safety workshops and simulator-based training.
