The altimeter is not measuring altitude
To understand the importance of a correct QNH setting, one must first realise what a classic barometric altimeter actually measures.
He does not know the altitude of the aircraft. Instead, he measures the static air pressure and converts this into an altitude reading using a specified atmospheric model.
As altitude increases, atmospheric pressure decreases. From this change in pressure, the instrument can derive a corresponding altitude. However, for this calculation to have a meaningful reference point, the pilot must set the correct reference pressure.
That is precisely what the QNH is for.
If the current QNH of an airfield is set, the altimeter will indicate approximately the airfield elevation above mean sea level. If the aircraft is in flight, the pilot correspondingly obtains a barometrically determined altitude above mean sea level.
If the set reference pressure is incorrect, the displayed altitude will also be wrong.
A few hectopascals can make a big difference
As a practical rule of thumb, it can be assumed that a deviation of one hectopascal causes a difference in altitude of approximately 27 to 30 feet.
At first, that doesn't sound like much.
With a ten-hectopascal difference, however, there is already a deviation of approximately 280 to 300 feet. At 20 hPa, this can become around 550 to 600 feet.
For a flight at high altitude and far from terrain, such a deviation might initially seem manageable. Close to the ground or during an instrument approach, the situation looks completely different.
An altitude error of several hundred feet can make the difference between adequate obstacle clearance and a dangerous proximity to terrain.
Even at the lower limit of a controlled airspace, an incorrect setting can cause a pilot to fly unintentionally higher than the display indicates.
„From high to low – look out below“
A mnemonic device known for decades describes one of the most important risks:
From high to low – look out below.
If an aircraft flies from an area of higher atmospheric pressure into an area of lower atmospheric pressure without adjusting the altimeter setting accordingly, it is actually lower than the altimeter indicates.
Precisely this situation is particularly dangerous.
For example, the pilot believes they are at 3,000 feet, whereas the actual barometric altitude is significantly lower.
The same basic principle also applies to significant temperature deviations from the standard atmosphere. Particularly at very low temperatures, the true altitude above ground may be lower than the indicated barometric altitude.
Therefore, low air pressure, cold temperatures and mountainous terrain deserve special attention.
EASA warns against incorrect altimeter settings
The European Union Aviation Safety Agency (EASA) has explicitly addressed the issue with the Safety Information Bulletin entitled „Incorrect Barometric Altimeter Setting“ and has since revised the publication.
The background is real-life events where incorrect or misunderstood altimeter settings led to significant deviations from the intended vertical profile.
This problem by no means affects only older aircraft with traditional round instruments.
Even modern glass cockpits cannot automatically correct an incorrectly entered reference pressure if the system does not have a corresponding automatic function or if an incorrect value was accepted by the pilot.
Modern avionics improve display quality, but they do not automatically make an incorrect input correct.
QNH, QFE and standard pressure
An additional source of error arises from the fact that different pressure references can be used.
For European flight operations, QNH and the standard pressure of 1013.25 hPa are particularly relevant.
Below the transition altitude, flying is normally conducted using QNH. Altitude information is then given as altitudes referred to mean sea level.
During the climb above the transition altitude, the altimeter is set to the standard pressure of 1013.25 hPa. From this point on, aircraft are no longer flown in altitudes, but in flight levels.
During descent, the transition is made in the opposite direction when passing through the transition level.
It is precisely this change that is one of the classic situations in which errors can occur.
The dangerous confusion of QNH and 1013
A particularly critical situation arises if the pilot accidentally remains on standard pressure during the descent.
Assuming the current QNH is only 983 hPa.
The difference to the standard pressure is approximately 30 hPa.
Multiplied by around 30 feet per hPa, this results in an altitude deviation of approximately 900 feet.
The aircraft could thus be nearly 1,000 feet away from the altitude the pilot expects based on their instrument display.
Such an error is already relevant during cruise flight. During an instrument approach or in mountainous terrain, it can become immediately dangerous.
Even small input errors can be problematic
It is not always a matter of spectacular differences of 20 or 30 hPa.
Even a misunderstood radio value can be sufficient.
A QNH of 1002 hPa, for example, can become 1022 hPa due to a hearing or input error. The difference is 20 hPa and thus already several hundred feet.
Such errors can occur, particularly when workload is high.
Typical situations are:
- Frequency change during descent
- heavy radio traffic
- deteriorations in the weather
- complex instrument approaches
- Runway changes
- short-notice ATC instructions
- severe turbulence
- Distractions caused by passengers or technical problems
In such situations, the seemingly trivial setting of the altimeter quickly becomes another item in a long line of tasks.
Cross-check altimeter before takeoff
One of the simplest and at the same time most effective checks takes place even before the start.
With the current QNH set, the altimeter should indicate approximately its published elevation on an airfield.
For example, if the aircraft is parked at an airfield with an elevation of 1,200 feet and the altimeter indicates only 700 feet after setting the current QNH, something is clearly wrong.
The discrepancy may indicate an incorrect QNH setting, a transmission error, or possibly an instrument problem.
This plausibility check takes only a few seconds and can detect an error before the aircraft even takes off.
Check again when changing frequency
Even during the flight, conscious monitoring is worthwhile.
A new QNH is transmitted, for example, during a frequency change, via ATIS or by the air traffic controller.
The value should not simply be adjusted mechanically.
It makes sense to carry out a brief mental cross-check:
Is the new value plausible?
How different is it from the previous QNH?
Is the change consistent with the weather conditions?
If the QNH suddenly changes by 15 or 20 hPa over a short distance, the question should at least arise as to whether the value was understood correctly.
When in doubt, checking with ATC is always the better solution.
Readback as a safety barrier
In controlled flight operations, radiotelephony forms an additional safety barrier.
If a QNH is transmitted, it should be read back carefully.
The purpose of the readback is not simply to comply with a formal requirement. It enables the air traffic controller to spot any misheard figures straight away.
Therefore, a QNH of 998 hPa must not casually become a „998“ that is subsequently transferred into the instrument without further attention.
You should be particularly vigilant when values are out of the ordinary.
Low air pressure warrants special attention
Many pilots are used to QNH values of between 1010 and 1025 hPa.
However, if a strong low-pressure area moves over Europe, significantly lower values can occur.
The more the QNH deviates from the standard pressure, the greater the consequences if standard pressure and QNH are mixed up.
Particularly unusual values should therefore serve as a mental warning marker.
A QNH of, say, 970 or 980 hPa is not just another number on the altimeter. It is also an indication that an incorrect setting can cause significant deviations in altitude.
Particular risk in the mountains
In mountainous terrain, altitude errors are naturally particularly critical.
If a pilot, for example, flies over a ridge with a planned terrain clearance of 1,000 feet, and the actual altitude error is already 400 or 500 feet, a significant portion of the safety margin has vanished.
Further factors may be added:
- strong winds
- downdrafts
- Turbulence
- poor visibility
- low cloud
- low temperatures
- Navigation deviations
The QNH setting is therefore an important part of overall risk management during Alpine flights or other flights in mountainous terrain.
Risk of airspace infringements
Not only terrain and obstacles make the subject relevant.
Airspace infringements can also be caused by incorrect altimeter settings.
For example, a pilot might intentionally remain 100 or 200 feet below the lower limit of controlled airspace.
If the QNH is set incorrectly, this small reserve can completely disappear.
The aircraft may then already be within controlled airspace, even though the altimeter apparently still shows a sufficient safety margin.
Especially beneath lowered airspace structures around larger airports, an additional reserve is therefore sensible.
Anyone wishing to avoid airspace with a lower limit of, for example, 2,500 feet should not necessarily fly underneath it at exactly 2,490 feet.
A sensible vertical buffer provides additional protection against measurement errors, turbulence and minor piloting errors.
Instrument approaches place particularly high demands
An incorrect QNH setting becomes especially critical during instrument approaches.
Many published minimum altitudes, Decision Altitudes and Minimum Descent Altitudes are based on a correct barometric altimeter setting.
An incorrect setting therefore effectively shifts the entire vertical profile.
The pilot may believe they are maintaining a published minimum altitude whilst the aircraft is actually significantly lower.
In poor visibility conditions, the problem is further exacerbated because there is no visual plausibility check against the terrain.
Precisely for this reason, altimeter cross-checks are an integral part of professional approach procedures.
Two altimeters are only better if both are checked
Many aircraft have two barometric altimeters.
This can be a classic mechanical altimeter alongside an electronic PFD, or two completely independent displays.
However, redundancy only helps if the adverts are actually compared with each other.
Anyone who enters the same incorrect QNH setting on both instruments will get two matching – but incorrect – altitude readings.
Therefore, it should not only be checked whether both instruments show the same altitude, but also whether the underlying pressure value has been set correctly.
Modern avionics do not automatically prevent the error
Glass cockpits easily give the impression, through their precise display, that the data shown must automatically be precise as well.
That is a dangerous fallacy.
A digital primary flight display can show an altitude to the exact foot. However, if the incorrect reference pressure is set, it merely displays an incorrect altitude with great precision.
The same applies to autopilots.
An autopilot reliably maintains the altitude dictated to it by the system. However, it cannot know that the pilot may have set the wrong barometric reference pressure.
Automation therefore changes the nature of the error, but does not eliminate it.
The most common causes are surprisingly banal
International safety reports repeatedly show similar starting points.
These include:
- QNH misheard
- Transposition of digits during entry
- used the QNH of another aerodrome
- Standard pressure set too early
- Standard pressure left too late during descent
- Change of QNH not applied
- lack of cross-checking between the instruments
- heavy workload
- Distraction
- incomplete cockpit procedures
It is noteworthy that no technical defect is required for such an error.
The entire altimeter system can function in a technically flawless manner – and yet, due to a single incorrect setting, it will show a significantly incorrect altitude.
Standardised procedures help
The most effective countermeasure is therefore not additional technology, but a consistent routine.
The change of an altimeter setting should be made consciously.
One possible scheme is:
Listen to QNH, read back, set and then cross-check the display.
For aircraft with two altimeters, the cross-check of both systems follows.
When switching to standard pressure or back to QNH, the change should also be carried out deliberately and verbalised if necessary.
Especially in two-pilot cockpits, this is one of the classic cross-checks. Single-pilot aviators can also benefit from comparable routines.
Do not cut personal safety margins too fine
At the same time, the QNH issue demonstrates why pilots should not necessarily exploit legal or published limits down to the very last foot.
Anyone flying beneath controlled airspace, travelling over mountainous terrain or required to maintain a minimum altitude benefits from an additional safety margin.
Such a reserve does not only protect against an incorrect QNH.
It takes into account simultaneously:
- Instrument deviations
- Turbulence
- minor tax errors
- Temperature effects
- Pressure changes
- Navigation inaccuracies
Professional airmanship often means precisely not flying right up to a limit.
A simple check with a big impact
The most important takeaway from the safety instructions is refreshingly simple.
The danger can be significantly reduced with just a few simple steps.
Before departure, the QNH is cross-checked against the aerodrome elevation. During the flight, new values are attentively received and read back. When transitioning between altitude and flight level, the altimeter setting is consciously checked. Unusual values lead to increased vigilance.
And if any information does not seem plausible, further enquiries will be made.
None of these measures takes a significant amount of time.
Conclusion
Setting the QNH is one of the most inconspicuous tasks in the cockpit. Precisely for that reason, it can easily become routine.
However, the physical consequences of an incorrect setting remain significant. A difference of just ten hectopascals can cause an altitude deviation of approximately 300 feet. In the case of larger errors, this can quickly amount to several hundred feet or even nearly 1,000 feet.
Close to the ground, in the mountains, during instrument approaches or immediately beneath controlled airspace, such a deviation can be critical.
Modern avionics can make many tasks easier, but they do not replace the critical eye of the pilot. An altimeter can only work with the reference pressure it is given.
Therefore, the most important rule remains remarkably simple: consciously set the QNH, read it back, check it for plausibility and monitor it regularly.
A small number in the altimeter window can determine a very large safety margin.
Source references:
Austro Control
