Ch 2 Quiz: Pressure & Altimetry — QNH, QFE, QNE & Errors

QNH is the local atmospheric pressure at Mean Sea Level, obtained by reducing the actual station pressure using the ISA temperature profile. With QNH set, the altimeter reads altitude above MSL.

QFE is the atmospheric pressure at the aerodrome reference point. With QFE set, the altimeter reads height above the aerodrome datum. The altimeter reads zero on the ground at the aerodrome.

Descending through the Transition Level (TL), the altimeter is changed FROM standard pressure (1013.25 hPa / QNE) TO local QNH. Ascending through the Transition Altitude, the change is from QNH to 1013.25 hPa.

High QNH means MSL pressure is above standard. The real atmosphere has a higher base pressure, so the actual air column height is GREATER than standard. True altitude is higher than indicated altitude.

The pressure altimeter uses the ICAO ISA temperature and pressure profile to convert sensed pressure into altitude. Any deviation from ISA conditions causes altimeter errors.

Near sea level, 1 hPa difference in pressure corresponds to approximately 27–30ft altitude error. This forms the basis of the altimeter correction calculation.

Difference = 1013.25 – 1003 = 10.25 hPa. Correction ≈ 10.25 × 27ft = 277ft. Since QNH is LOWER than 1013.25, the aircraft is LOWER than indicated. Actual altitude ≈ 5,000 – 277 ≈ 4,723ft ≈ 4,700ft.

Pressure altitude is the altitude indicated when the altimeter sub-scale is set to 1013.25 hPa. It is expressed as Flight Levels (e.g., FL250 = pressure altitude 25,000ft).

In colder than ISA conditions, the atmosphere is denser and pressure levels are compressed. The aircraft is at a LOWER true altitude than indicated. Remember: ‘High, Hot, High’ / ‘Cold, Low, Low’.

Cold temperature error causes the aircraft to be at a LOWER true altitude than indicated. This is critical for terrain clearance — ICAO PANS-OPS has cold temperature corrections for instrument approaches.

QNE is the standard pressure setting of 1013.25 hPa. When set on the altimeter, it gives Flight Levels (pressure altitude). Used above the Transition Level in India and internationally.

Both aircraft have the same pressure altitude (FL350), but QNH differs between locations. The actual true altitude above MSL depends on local sea level pressure, so their true altitudes will differ.

In cold temperatures, the aircraft is LOWER than indicated. ICAO cold temperature corrections increase the altitude to fly to compensate. Flying the uncorrected altitude risks terrain impact.

A blocked static port freezes the altimeter reading at the last captured altitude — the aircraft has no valid altitude indication at all, making it the most dangerous error. A blocked static significantly affects the altimeter, ASI, and VSI.

India’s transition altitude is 5,000ft AMSL or aerodrome elevation + 500ft, whichever is higher. Below the TA: QNH is used. Above the Transition Level (TL): 1013.25 hPa (QNE/FL).

Near sea level, pressure decreases at approximately 1 hPa per 27–28ft, which is approximately 36–37 hPa per 1000ft, or about 3.7 hPa per 100m. The 27ft/hPa approximation is standard.

Turning the sub-scale knob clockwise INCREASES the pressure setting, which causes the indicated altitude to INCREASE. Setting a higher QNH than 1013.25 hPa will increase the altimeter reading.

Difference: 1013.25 – 993 = 20.25 hPa × 27ft = 547ft. Since QNH is lower than standard, aircraft is LOWER than indicated. True clearance ≈ 1,000 – 547 ≈ 453ft ≈ 460ft. Dangerously close to terrain!

Density altitude = pressure altitude adjusted for temperature. High density altitude (high temp, high elevation, low pressure) reduces air density, degrading engine power, propeller/rotor thrust, and aerodynamic lift.

If QNH is set too LOW (lower than actual), the altimeter reads HIGHER than actual altitude. Similarly, flying from high to low pressure (without resetting) causes over-reading (‘From high to low, look out below’).

The pitot tube measures total (stagnation) pressure. The static port measures ambient pressure. The difference is dynamic pressure q = ½ρV², which is used by the airspeed indicator to compute indicated airspeed.

A blocked static port means the altimeter (which senses static pressure) will be frozen at the last reading when the blockage occurred. It will not change with actual altitude changes.

True altitude equals indicated altitude only when: exact ISA conditions exist (actual temperature = ISA temperature throughout the column), AND the correct QNH is set on the altimeter sub-scale.

Barometric VNAV uses altimetry. In cold temperatures, the aircraft is lower than the altimeter indicates. On a geometric descent path, this means the aircraft is below the safe VNAV path — critical for obstacle clearance.

With QNH set correctly (1025 hPa in this case), the altimeter reads the aerodrome elevation above MSL, which is 1,000ft. The setting of QNH normalises the altimeter to show MSL altitude.

Geopotential altitude accounts for the slight decrease of gravity with height. At low altitudes the difference is negligible, but at very high altitudes (above 80km), geopotential altitude is slightly less than geometric altitude.

ICAO cold temperature corrections increase all altitude constraints (minimums, crossing altitudes) to ensure the aircraft clears obstacles. DGCA/ICAO PANS-OPS provides correction tables.

Pressure height/altitude = altitude indicated when altimeter sub-scale is set to 1013.25 hPa. Used for performance calculations in aircraft manuals, expressed as FL above the transition level.

Flying from HIGH to LOW pressure at constant indicated altitude: the aircraft descends relative to MSL. ‘Flying from HIGH to LOW, look out below.’ The standard mnemonic for altimetry safety.

The radio altimeter (RA) uses radar signals reflected from the ground to measure actual height above ground level. Unlike the barometric altimeter, it is not affected by pressure, temperature, or altimeter setting errors.

QNH IS the sea level pressure. QNH 1033 hPa means the atmospheric pressure at MSL is 1033 hPa on that day. It is approximately 20 hPa above standard (1013.25 hPa).

The semi-circular cruising level rule assigns odd thousands (500ft) to certain track directions and even thousands to others based on magnetic track. This ensures vertical separation between opposite-direction traffic.

10 hPa below standard × 27ft/hPa = 270ft. When the pilot correctly sets QNH 1003, the altimeter reads TRUE altitude. The pilot MUST use QNH for approach.

A blocked static port in flight is not correctable by the pilot without the alternate static source. Setting errors, pressure errors, and cold temperature errors all have pilot-applicable corrections.

Safe altitudes in mountainous terrain must account for: Minimum Safe Altitude (MSA), cold temperature corrections (especially in winter), obstacle clearance, and altimeter accuracy. DGCA procedures mandate these corrections.

At FL410 (41,000ft / 12,497m, above the tropopause), using ISA equations, pressure is approximately 175 hPa. Pressure at FL360 (tropopause) is ~226 hPa, and pressure continues to decrease above.

QFE shows height above the aerodrome, but terrain between aerodromes may be higher. International operators use QNH (MSL-based) to maintain consistent reference above MSL, especially in mountainous terrain.

Lag error occurs because the aneroid capsule and mechanical linkages in the altimeter take finite time to fully respond to rapid pressure changes, particularly during fast descents or climbs.

The Transition Level is the first available FL at or above the Transition Altitude (TA). It must be at least 300ft (in ICAO standard) above the TA to ensure no overlap between QNH altitudes and FLs.

When flying at Flight Levels (altimeter set to 1013.25 hPa), QNH changes below do not affect the FL readout. However, the TRUE altitude decreases when QNH is below 1013.25 hPa.

ICAO standards specify altimeter accuracy within ±20ft at sea level. However, all altimeters accumulate errors with temperature deviations, instrument hysteresis, and calibration drift.

If the sub-scale is set 15 hPa TOO LOW, the reference is below actual sea level pressure. The altimeter UNDER-reads — it shows LESS altitude than actual. Error = 15 × 27ft = 405ft lower. Dangerous over terrain!

During the Indian monsoon, mesoscale convective systems (MCS) can cause rapid, localised pressure changes. If QNH is not updated frequently, altimetry errors can develop, especially in convective areas.

The altimeter is calibrated using ISA temperature profile to convert pressure to altitude. If actual temperatures differ (warmer or colder than ISA), the actual altitude column height differs from the ISA model, causing true altitude to differ from indicated.

RVSM reduces vertical separation from 2000ft to 1000ft between FL290 and FL410. This requires calibrated, accurate altimeters (within ±75ft), automatic altitude control, and TCAS. Indian airspace applies RVSM standards.

Density altitude = pressure altitude + (ISA deviation × ~120ft/°C approximately). ISA+25 with a positive pressure altitude gives high density altitude. Performance is degraded — longer takeoff roll, reduced climb rate.

Below India’s transition altitude, the current local QNH (updated every 30 minutes or more frequently in rapidly changing conditions) must be set. This ensures altimeters read altitude above MSL accurately for terrain clearance.

MSA provides 300m (1000ft) obstacle clearance within a 46km (25NM) radius of a specified navigation facility. It is published on approach charts and used for emergency/safety purposes, not for normal routing.

Magnetic variation affects compass headings, not altimeters. Altimeter errors include: instrument error (calibration), barometric error (wrong QNH/QNE set), temperature/density error, and lag error.

Difference: 1013.25 – 982 = 31.25 hPa × 27ft = 844ft. Since 1013.25 > QNH, altimeter reads HIGHER than actual by 844ft. Actual elevation 1500ft → altimeter reads ≈ 1500 + 844 ≈ 2344ft ≈ 2310ft.