CompressorsAircraft & Engines — DGCA CPL practice questions

Question 1 of 5

A single stage of a centrifugal compressor can achieve a compression ratio of about:

A.1.2:1

B.4:1

C.12:1

D.30:1

All 5 questions — Compressors

Aircraft & Engines · DGCA CPL. The correct option is marked on each.

Q1. A single stage of a centrifugal compressor can achieve a compression ratio of about:
- A.1.2:1
- B.4:1✓
- C.12:1
- D.30:1
Why: Centrifugal: robust, large frontal area, ~4:1 per stage. Axial stages give only ~1.2:1 each but can be stacked to 30:1 or more.
Q2. In an axial compressor, the stators:
- A.Accelerate the air
- B.Form divergent passages that convert velocity into pressure✓
- C.Spin with the rotor
- D.Cool the turbine
Why: Each stage: rotating blades accelerate the air, then stationary stator vanes (divergent passages) slow it, raising pressure.
Q3. A compressor surge is:
- A.A small RPM fluctuation
- B.A complete breakdown of airflow through the compressor, often with a loud bang✓
- C.Normal behaviour at takeoff power
- D.An oil system fault
Why: In surge, high-pressure air from the rear bursts forward out of the intake — thrust loss, rapid EGT rise, possible damage.
Q4. Compressor bleed valves are opened at low RPM to:
- A.Cool the cabin
- B.Increase axial velocity in the front stages and prevent stall✓
- C.Boost thrust
- D.Reduce EGT at takeoff
Why: Bleeding air from the middle stages keeps the front stages' airflow matched to their blade speed, preventing stall during low RPM and acceleration.
Q5. Multi-spool engines exist primarily to:
- A.Reduce engine weight
- B.Let each compressor section run at its own optimal speed across the RPM range✓
- C.Simplify maintenance
- D.Drive two propellers
Why: LP/IP/HP spools each driven by their own turbine avoid stall and keep the whole compressor efficient over a wide operating range.