Pitot and Static SourcesNavigation — Instrumentation — DGCA CPL practice questions
Question 1 of 8
A pitot head is used to measure:
All 8 questions — Pitot and Static Sources
Navigation — Instrumentation · DGCA CPL. The correct option is marked on each.
Q1. A pitot head is used to measure:
- A.Dynamic minus static pressure
- B.Static plus dynamic pressure✓
- C.Static pressure
- D.Dynamic pressure
Why: The pitot head is an open-ended tube that brings the moving air to rest inside. This generates the dynamic component on top of the existing static pressure. The head senses their sum: total (pitot) = static + dynamic. It does not subtract — that is done by the instrument capsule.
Q2. A static vent is used to measure:
- A.Dynamic pressure minus static pressure
- B.Dynamic pressure plus static pressure
- C.Atmospheric pressure✓
- D.Dynamic pressure
Why: The static vent opening is at right angles to the airflow so no dynamic component is sensed. It measures only ambient (atmospheric/static) pressure at that altitude. "Atmospheric pressure" and "static pressure" are synonymous here.
Q3. A pressure head is subject to the following errors:
- A.Position, manoeuvre-induced, temperature
- B.Position, manoeuvre-induced✓
- C.Position, manoeuvre-induced, density
- D.Position, manoeuvre-induced, instrument
Why: The pressure head (the sensing assembly) has exactly two errors: position error (incorrect static sensing due to turbulence, speed, attitude) and manoeuvre-induced error (transient fluctuations during pitch/flap/gear changes). Temperature, density, compressibility, and instrument errors belong to the individual instruments, not the head. Rule: Pressure head errors = position + manoeuvre-induced. Only two. Everything else is an instrument error.
Q4. Given: P t = total pressure; P s = static pressure. Dynamic pressure is:
- A.P t − P s✓
- B.(P t − P s ) / P t
- C.(P t − P s ) / P s
- D.P t / P s
Why: From the fundamental relationship: Pitot = Static + Dynamic → Dynamic = Pitot − Static = P t − P s . Simple rearrangement. The ASI and Machmeter capsule does exactly this subtraction internally.
Q5. Manoeuvre-induced error:
- A.Is caused by pressure changes at static probes or vents✓
- B.Is likely to be greatest when yawing after engine failure
- C.Is combined with instrument and position error on a correction card
- D.Lasts for only a short time at high altitude
Why: Manoeuvre-induced error is directly caused by short-term pressure fluctuations at the static vents and delays in the associated pipelines. Pitch attitude changes, flap and gear deployment are the prime triggers.
Q6. Position error:
- A.May be reduced by the fitting of static vents✓
- B.Will usually decrease with an increase in altitude
- C.Will depend solely on the attitude of the aircraft
- D.Will usually decrease as the aircraft approaches the speed of sound
Why: Static vents are placed at fuselage locations where airflow is less turbulent, giving a better approximation of true static pressure. Cross-balancing vents on both sides reduces yaw-induced errors further. "Reduce" not "eliminate" — position error still exists, just smaller.
Q7. Fitting static vents to both sides of the aircraft fuselage will:
- A.Reduce the position error
- B.Balance out errors caused by side-slipping or yawing✓
- C.Require a calibration card for each static vent
- D.Enable a greater number of instruments to be fitted
Why: When the aircraft yaws or side-slips, one side sees higher pressure and the other lower. By cross-connecting vents on both sides, the pressures are averaged and the yaw/sideslip error largely cancels out. This is called cross-balancing .
Q8. Where an alternate static source is fitted, use of this source usually leads to:
- A.A temporary increase in lag error
- B.A lower pressure error than with normal sources
- C.An increase in position error✓
- D.No change in position error
Why: The primary static vent is at the optimum location; its position error is well characterised and accounted for. The alternate source is not at this optimum position — its position error is greater. Additionally, alternate static pressure is likely lower than ambient (aerodynamic suction), further increasing the error. Practical note: On selecting alternate static expect: increased position error; instruments likely to over-read altitude/airspeed slightly. Check the Operating Data Manual for specific correction values.