Altimeter, Pressure Altitude & QNH

C. continue to show the reading at which the blockage occurred

The altimeter measures static pressure only. With the source blocked, the pressure in the case stays as it was, so the reading freezes - in a climb or a descent.

The question was recalled without its options; these options were written to it.

A. 970 hPa

QFE is the pressure at the aerodrome; QNH is that pressure reduced to sea level. Going up 1200 ft loses 1200/30 = 40 hPa: QFE = 1010 - 40 = 970 hPa.

A. 6300 ft

FL75 is 7500 ft on 1013.25. QNH 1023.25 is 10 hPa higher = 300 ft at 30 ft/hPa, so true altitude is 7800 ft; less 1500 ft aerodrome elevation = 6300 ft.

A. 1008 hPa, increasing by 8 hPa

The correct answer is: 1008 hPa, increasing by 8 hPa.

A. electromagnetic pick-off

A servo altimeter is more accurate mainly because of its electromagnetic (induction) pick-off.

C. 6300 ft

Aerodrome pressure altitude = 1500 + (1013 - 1023) x 30 = 1200 ft. To reach FL75 (7500 ft) it must climb 7500 - 1200 = 6300 ft.

D. at high speed the non-compensated altimeter shows a higher altitude

Position error grows with speed and usually makes the static source sense a lower pressure than ambient, so the uncorrected altimeter over-reads (shows higher) at high speed. Mode C reporting comes from the corrected (ADC) source.

The question was recalled without its options; these options were written to it.

A. Over read

Cold air is denser, so pressure drops faster with height: the altimeter, calibrated for ISA, over-reads in colder air (when flying from warm to cold, from high to low).

A. the aerodrome elevation is unchanged; its pressure altitude falls by about 270 ft

Elevation is a fixed height above sea level. Higher pressure means the 1013 hPa level lies lower, so the aerodrome's pressure altitude drops by 9 x 30 = 270 ft; an altimeter left on 1013 would read 270 ft less.

A. (i) vacuum (or a very low pressure), (ii) static pressure

The capsule is evacuated (a very low pressure); static pressure surrounds it in the sealed case.

A. altitude in the standard atmosphere at which the prevailing density is equal to the density in the standard atmosphere

Density altitude is defined as the altitude in the standard atmosphere at which the prevailing density occurs. Pressure altitude corrected for temperature is the practical way to work it out, but the definition asked for is (a).

C. high elevation, high temperature, high humidity

Density altitude rises with height (low pressure), heat and humidity (water vapour is lighter than dry air). "Hot, high and humid" is the worst for performance.

Our independent check found that more than one option can be argued for, as worded in the recall. The key gives the answer most references support.

The question was recalled without its options; these options were written to it.

B. when approaching the selected altitude and when deviating from it after capture

It gives an alert as the aircraft approaches the selected altitude (typically about 1000 ft before) and again if it then departs from it (typically by more than 200-300 ft).

The question was recalled without its options; these options were written to it.

B. Under indicate

Warmer air is less dense per height, so true altitude is higher than indicated: the altimeter under-indicates.

B. True altitude is lower

Colder air makes the true altitude lower than the indicated altitude.

B. QNH

QNH -> altitude above mean sea level (reads elevation on the ground). QFE -> height above the aerodrome (reads zero). 1013.25 (standard) -> pressure altitude / flight level; QNE is the reading on 1013.25 at the aerodrome.

The question was recalled without its options; these options were written to it.

B. Time at given altitude

Altimeter hysteresis (the capsule slowly relaxing after a height change) depends on how long the altimeter has been at that altitude, not on Mach number.

B. 15200

ISA at 16,000 ft is -17 C; at -30 C it is 13 C colder, so true altitude is lower than indicated by about 4 ft x 13 x 16 = 830 ft: about 15,200 ft.

B. 5550 ft

ISA at 5000 ft is +5 C, so the air is ISA + 30: warm air expands, so you are higher than indicated - roughly 4 % per 10 C: 5000 x 308/278 = about 5540 ft.

The question was recalled without its options; these options were written to it.

A. -210

Pressure altitude of the aerodrome = (1013 - QFE) x 30 = (1013 - 1020) x 30 = about -210 ft; the elevation does not enter.

A. It will read higher than actual altitude

Flying from high to low pressure without resetting, the altimeter over-reads: it shows higher than the true altitude.

A. 27,500 ft

ISA at 29,000 ft = 15 - 58 = -43 C, so ISA -12. Density altitude = 29,000 - 12 x 120 = 27,560, about 27,500 ft.

The question was recalled without its options; these options were written to it.

A. position pressure error

Error from varying static pressure at the static source is position (pressure) error.

C. 10200

ISA at 10,000 ft is -5 C; 0 C is ISA +5, so the air is warmer and true altitude is higher: about +4 ft x 5 x 10 = +200 ft.

B. 24 min / 60 NM

Time = 12,000 / 500 = 24 min. Mean TAS at the mid level (10,000 ft, ISA - 5) is about 150 kt: 150 x 24/60 = 60 NM.

B. pressure altitude

Pressure altitude is the reading on the standard setting; flight levels are pressure altitude in hundreds of feet.

The question was recalled without its options; these options were written to it.

A. +/- 30 ft

Typical tolerances at sea level: simple altimeter about 100 ft, sensitive about 70 ft, servo about 30 ft.

The question was recalled without its options; these options were written to it.

B. static pressure

The altimeter compares static pressure with the ISA pressure-height law - it never measures distance to the ground; that is the radio altimeter.

The question was recalled without its options; these options were written to it.

A. pressure, temperature and density all decrease (temperature only up to the tropopause)

Pressure and density fall continuously; temperature falls about 2 C/1000 ft to the tropopause, then stays constant.

B. a stack of evacuated capsules, jewelled bearings and a subscale, with two or three pointers

More capsules give more movement per foot, jewelled bearings cut friction, and a subscale lets the pilot set QNH/QFE/1013.

C. Reduce the effect of friction in the linkages

The vibrator overcomes friction (stiction) in the linkage so the pointer does not stick.

A. Accurate, electric pick off coil

A servo altimeter is more accurate because an electrical pick-off (induction) coil replaces mechanical linkage.

A. 25400 ft

Pressure altitude = elevation + (1013 - QNH) x 30 = 25,000 + 14 x 30 = 25,420 ft, about 25,400 ft.

B. vacuum / static pressure

The capsule is evacuated and the case receives static pressure.

B. 1200 ft higher

Flying from high pressure to low pressure with the setting unchanged, the altimeter over-reads: 40 hPa x 30 ft = 1,200 ft higher than the true altitude.

B. 6400FT

QFE 963 means the aerodrome is at 963 hPa; on 1013 that is about 50 hPa x 27 ft = 1,350-1,500 ft. Add 5,000 ft: about 6,400 ft.

C. The International Standard Atmosphere.

Pressure altimeters are calibrated to the International Standard Atmosphere.

A. to indicate zero feet

With the static vents sealed the pressure is trapped at the ground value, so with QFE set the altimeter stays at zero as the aircraft climbs.

A. -600 ft

Density altitude = pressure altitude + 120 ft per degree of ISA deviation: 0 + 120 x (-5) = -600 ft. Cold air is dense - the aircraft performs as if below sea level.

The question was recalled without its options; these options were written to it.

C. FL120

Minimum altitude = 10,600 + 1,500 = 12,100 ft. QNH is 10 hPa above standard, so the 1013 datum lies about 300 ft higher: pressure altitude = 12,100 - 300 = 11,800 ft. The lowest flight level not below that is FL120. (Forgetting the QNH correction gives FL130 or FL110.)

The question was recalled without its options; these options were written to it.

A. Static pressure

The aneroid capsule is evacuated and senses the static pressure of the surrounding case; it expands as static pressure falls.

A. 4690 ft

(1013 - 990) x 30 = 690 ft. Pressure below standard, so add: PA = 4000 + 690 = 4690 ft.

C. 9280 ft

PA = 5500 + (1013 - 995) x 27 = about 5990 ft. ISA there = 15 - 12 = +3 C, so ISA + 27. DA = 5990 + 27 x 120 = about 9230 ft.

The question was recalled without its options; these options were written to it.

A. under-read

In a right sideslip the air strikes the right side, raising the pressure at the right vent. With the left (low-pressure) vent blocked the two can no longer average out, the system senses too high a pressure, and the altimeter reads too LOW.

The question was recalled without its options; these options were written to it.

C. instrument, position, manoeuvre-induced, temperature, barometric, lag

Altimeter: instrument, position (pressure), manoeuvre-induced, temperature, barometric (wrong datum) and time lag. Density and compressibility errors belong to the ASI.

The question was recalled without its options; these options were written to it.

A. the QNH increases

QNH is the field pressure reduced to sea level with a fixed correction for elevation: if QFE rises by 21 hPa, QNH rises by the same 21 hPa. Elevation never changes.

The question was recalled without its options; these options were written to it.

A. static (starting) friction in the mechanism

Small pressure changes may not overcome the friction of the linkage (stiction); a vibrator keeps the mechanism moving so it responds smoothly.

The question was recalled without its options; these options were written to it.

C. 1675 ft

Lower pressure than standard means the 1013 level is below sea level: PA = 1000 + (1013 - 988) x 27 = 1000 + 675 = 1675 ft.

The question was recalled without its options; these options were written to it.

B. decreases by 8 hPa

A's QNH = 1000 + 240/30 = 1008 hPa; B's QNH = 1000 hPa. Sea-level pressure falls 8 hPa from A to B (4 hPa by the mid-way point) - "high to low, beware below": with A's setting the altimeter over-reads.

A. -270

With 1013 set the altimeter reads pressure altitude. At a sea-level aerodrome with QNH 1022 that is (1013 - 1022) x 30 = -270 ft.

As recalled, this question depended on a chart or table that is not reproduced here; the key is the recalled paper's answer.

A. 630 ft

Pressure altitude depends only on the pressure there: (1013 - 992) x 30 = +630 ft (above the standard datum).

The question was recalled without its options; these options were written to it.

A. 993 hPa

PA is 600 ft above elevation, so the pressure is 600/30 = 20 hPa below standard: QNH = 1013 - 20 = 993 hPa.

The question was recalled without its options; these options were written to it.

A. 800 ft

With 1013 set where the QNH is 1003, the altimeter shows 10 x 30 = 300 ft too high: 500 + 300 = 800 ft. Forgetting to reset into low pressure makes you lower than you think.