The Pressure AltimeterNavigation — Instrumentation — DGCA CPL practice questions

All 6 questions — The Pressure Altimeter

Navigation — Instrumentation · DGCA CPL. The correct option is marked on each.

1. Q1. In the International Standard Atmosphere, the mean sea level pressure is ......., the lapse rate of temperature ....... between MSL and ....... and is isothermal up to ........ The numbers missing are:

   • A.1225 hPa; 2° per 1000 ft; 37 000 ft; 66 000 ft
   • B.1013.25 hPa; 1.98°C per 1000 ft; 36 090 ft; 65 617 ft✓
   • C.1013.25 hPa; 1.98°C per 1000 ft; 36 090 ft; 104 987 ft
   • D.1225 hPa; 1.98°C per 1000 ft; 36 090 ft; 104 987 ft

   Why: ISA MSL pressure = 1013.25 hPa . Temperature lapse rate = 1.98°C per 1000 ft from MSL to tropopause at 36 090 ft . Isothermal layer (constant −56.5°C) extends from 36 090 ft to 65 617 ft (11–20 km). See Section 3 . — Common trap: 1225 is density (g/m³); 1013.25 is pressure (hPa). The isothermal layer ends at 65 617 ft (20 km), not at 104 987 ft (32 km) which is the top of the upper stratosphere.

2. Q2. An aircraft taking off from an airfield with QNH set on the altimeter has both static vents blocked by ice. As the aircraft climbs away the altimeter will:

   • A.Read the airfield elevation✓
   • B.Indicate the aircraft height AMSL
   • C.Read the height of the aircraft above the airfield
   • D.Show only a very small increase in height

   Why: Both static vents are blocked at the moment of take-off (at airfield elevation). With QNH set, the altimeter reads the aerodrome elevation on the ground. When both static vents ice over on the ground and the aircraft climbs, the static pressure in the instrument is frozen at the airfield level value. The altimeter continues to display the altitude that corresponds to the airfield elevation (whatever QNH altitude that was at ground level) — it cannot respond to the changing external pressure. See Section 13 . — Blocked static at ground level = frozen at the pressure existing at the moment of…

3. Q3. When flying from low pressure to high pressure, without resetting the altimeter datum, the barometric error of an altimeter will cause the instrument to:

   • A.Read the true altitude, providing a correction is made for temperature
   • B.Over-read the true altitude of the aircraft
   • C.Indicate a higher altitude than the correct one
   • D.Under-read the true altitude of the aircraft✓

   Why: Flying from LOW to HIGH pressure without resetting is the opposite of the dangerous scenario. At the destination where MSL pressure is higher, the 1025 hPa datum set is now ABOVE the actual MSL level at the destination. The altimeter reads height above a datum that is above MSL → it under-reads true altitude (the aircraft is actually higher than indicated). The aircraft is safely above terrain — this is the benign error direction. The dangerous case is HIGH to LOW pressure (altimeter over-reads = aircraft lower than shown). See Section 10 . — Memory aid: HIGH to LOW (pressure or temperature…

4. Q4. The errors affecting the pressure altimeter are:

   • A.Instrument position, manoeuvre induced, density, temperature, lag
   • B.Instrument, pressure, manoeuvre induced, density, temperature, lag
   • C.Instrument, position, manoeuvre induced, temperature, barometric, lag✓
   • D.Instrument, pressure, lag, barometric, temperature, compressibility

   Why: The six altimeter errors are: instrument error, position (pressure) error, manoeuvre-induced error, temperature error, barometric error, and time lag. Note that "density error" is an ASI error (not altimeter); "compressibility error" is also an ASI error. The altimeter has barometric error (from incorrect subscale setting) and temperature error (from ISA deviation). See Section 10 . — Altimeter errors: I-P-M-T-B-L (Instrument, Position, Manoeuvre-induced, Temperature, Barometric, Lag). Density and compressibility errors belong to the ASI, not the altimeter.

5. Q5. An altimeter with ....... set on the subscale will indicate ......., but with ....... set, the altimeter will show .......

   • A.1013; pressure altitude; QNH; altitude✓
   • B.QNE; pressure altitude; QNH; height above airfield datum
   • C.QFE; height above the airfield datum; 1013; height AMSL
   • D.QNH; height above touchdown; 1013; height AMSL

   Why: Setting 1013 (hPa) on the subscale = pressure altitude reading. Setting QNH = altitude AMSL reading. These are the two fundamental altimeter datum settings in the options. See Section 12 . — The clean mapping: 1013.25 hPa → Pressure Altitude (Flight Levels); QNH → Altitude AMSL; QFE → Height above aerodrome (zero on ground). This is fundamental and exam-frequent.

6. Q6. An aircraft has one altimeter set to QFE and one to aerodrome QNH 1000 hPa. If the airfield elevation is 300 ft, immediately before take-off the altimeter with QFE set will read ....... and the other ....... If the QFE altimeter is set to 1013 when passing through the transition altitude 3000 ft, it will read ..... (Assume 1 hPa = 30 ft).

   • A.300 ft; zero; 2610 ft
   • B.Zero; 300 ft; 3390 ft✓
   • C.Zero; 300 ft; 3690 ft
   • D.Zero; 300 ft; 2610 ft

   Why: Part 1 — Before take-off: QFE set altimeter reads zero (by definition, QFE is aerodrome level pressure). QNH set altimeter reads 300 ft (aerodrome elevation). Part 2 — At transition altitude 3000 ft (QFE altimeter, then reset to 1013): At 3000 ft on QFE, the QFE altimeter reads 3000 ft (height above aerodrome). The aircraft is at 3000 ft + 300 ft elevation = 3300 ft pressure altitude relative to QNH 1000 hPa. Pressure difference between QNH 1000 and standard 1013: 1013 − 1000 = 13 hPa × 30 ft = 390 ft. 1013 hPa datum is 390 ft LOWER than 1000 hPa level. So with 1013 set, the indicated altit…