Learning objective: You map a blockage (drain / ram / static) to the right lie — and you split IAS→CAS (instrument, AFM) from CAS→TAS (density, not “the ASI is broken”).
60 seconds: You are flying a climb in IMC, pitot heat on, the ASI climbs with altitude, altimeter and VSI still look sane. That is pitot and drain blocked, static free: trapped pt, ASI like an altimeter. Remember: drain vs ram vs static first — or the exam answer is wrong.
Four “errors” in the syllabus. Only the first two are really instrument defects:
| What | What you do |
|---|---|
| Instrument + position | IAS → CAS from the AFM table |
| Density | CAS → TAS (CRP-5) — the ASI is not broken |
| Compressibility | CAS → EAS; PPL teaching: higher Mach/FL |
| Blockage | no correction factor — the instrument lies |
Rule of thumb (teaching, not AFM): TAS ≈ CAS + (2 % × altitude in 1000 ft). Proper TAS needs temperature on the CRP-5.
Exam matrix (teaching, not AFM):
- Pitot blocked, drain open, static free: ASI ≈ 0.
- Pitot and drain blocked, static free: ASI like ALT — climb high, descent low; ALT/VSI normal.
- Static blocked, pitot free: ASI low in climb / high in descent; ALT frozen; VSI ≈ 0. Fix: alternate static (teaching: cabin slightly lower pressure → slight over-read of height/speed — type AFM).
- Pitot and static blocked: ASI and ALT hold the last value, VSI ≈ 0.
- Insect/cover on the ground: IAS stays ~0. No wasp overpressure.
Pitot heat: on before ice seals the tube. Current draw and OAT bands are type AFM, not a universal amp figure.
GPS groundspeed versus IAS is only rough — wind sits in between. And a dead ASI with no gyro flag is pitot-static, not an OFF flag.
“A wasp nest makes overpressure, so the ASI reads low.” That is wrong. People hear “too low” and invent the physics. On the ground the trapped pressure is ambient — on the roll, q simply never rises.
Exam knowledge
- Pitot+drain blocked, static free: ASI like ALT (climb high / descent low).
- Drain open with pitot blocked: ASI ≈ 0.
- Static blocked: ASI climb low / descent high; ALT frozen; VSI 0. Alternate static.
- Both blocked: last values, VSI ≈ 0.
- IAS→CAS = instrument+position (AFM). CAS→TAS = density, not a fault.
- Insect on the ground → IAS ~ 0.
Source: typical EASA PPL 020 blockage teaching / FAA AFH. Matrix is exam teaching, not AFM. Heat amps and V-speeds only from the type.
Check yourself
1. Pitot blocked, static open. How does the ASI behave in climb and descent?
- A) ASI frozen on last value; altimeter and VSI zero
- B) ASI acts like an altimeter: high in climb, low in descent
- C) ASI low in climb, high in descent; altimeter frozen
- D) Only the VSI is affected
2. Static blocked, pitot open. Which combination is correct?
- A) ASI like an altimeter; altimeter and VSI normal
- B) All three unreliable; alternate static does nothing
- C) ASI frozen; altimeter keeps climbing
- D) ASI low in climb / high in descent; altimeter frozen; VSI zero
- B — pitot blocked / static free: ASI like an altimeter.
- D — static blockage. A is pure pitot blockage. Alternate static is the fix.