Cessna 172N Aircraft Systems — PPL Oral Exam Questions

DPE oral questions · aircraft systems

8 questions a DPE may ask in this section

1. Question 1 · PA.I.G.K1

   Explain how a carburetor works and when carb ice is most likely to form.

   What a DPE expects to hear

   • Carburetor mixes fuel and air; venturi causes pressure drop which causes temperature drop
   • Temperature drop of 30-40°F can occur inside carburetor
   • Ice can form even when outside air temperature is 70°F
   • Most likely in visible moisture (clouds, fog, rain) or high humidity at 20-70°F OAT
   • Most critical at low power settings (approach, descent)
   • Carb heat uses heated air from exhaust manifold — introduces unfiltered air

   Common wrong answers

   • Thinking carb ice only forms when it's cold outside
   • Not knowing carb heat introduces unfiltered air

   SourcePHAK FAA-H-8083-25C Chapter 7; AFH Chapter 2

2. Question 2 · PA.I.G.K1

   Describe the electrical system on this aircraft. What happens if the alternator fails?

   What a DPE expects to hear

   • Engine-driven alternator charges battery and powers electrical bus
   • Battery provides backup power if alternator fails
   • Alternator failure symptoms: ammeter shows discharge, low-voltage light illuminates
   • Actions: shed non-essential electrical loads, land as soon as practical
   • Battery-only time is limited — typically 30-60 minutes depending on load

   SourcePHAK FAA-H-8083-25C Chapter 7; Aircraft POH Chapter 7 (Abnormal Procedures)

3. Question 3 · PA.I.G.K1

   How does the pitot-static system work, and which flight instruments does it power?

   What a DPE expects to hear

   • Pitot tube measures ram air pressure (impact pressure)
   • Static ports measure ambient atmospheric pressure
   • Airspeed indicator: uses both pitot and static (differential)
   • Altimeter: uses static only
   • Vertical Speed Indicator (VSI): uses static only
   • Pitot heat prevents ice blockage of pitot tube

   SourcePHAK FAA-H-8083-25C Chapter 8

4. Question 4 · PA.I.G.K1

   Explain the gyroscopic instruments — which are vacuum-driven and which are electrically driven?

   What a DPE expects to hear

   • Vacuum-driven (typical): Attitude Indicator (AI), Heading Indicator (DI)
   • Electrically driven (typical): Turn Coordinator (or Turn and Bank Indicator)
   • Some aircraft are different — student should verify for their aircraft
   • Gyroscopic instruments require warmup time
   • Vacuum system uses engine-driven vacuum pump

   SourcePHAK FAA-H-8083-25C Chapter 8

5. Question 5 · PA.I.G.K2

   What is the purpose of the fuel selector, and what is correct fuel management for a typical training flight?

   What a DPE expects to hear

   • Fuel selector controls which tank supplies fuel to the engine
   • For aircraft with BOTH position (172S): typically left on BOTH for all operations
   • For aircraft without BOTH (Piper Warriors): must switch tanks periodically — typically every 30 minutes
   • Always on BOTH (or appropriate tank) for takeoff and landing
   • Student should reference THEIR specific POH for fuel management procedure

   SourceAircraft POH Section 4 (Normal Procedures)

6. Question 6 · PA.I.G

   If the static port becomes completely blocked by ice during our climb, what happens to your three pitot-static instruments?

   What a DPE expects to hear

   • Altimeter freezes at altitude where blockage occurred
   • VSI returns to zero
   • ASI inaccurate (reads low in climb, high in descent)

   SourcePHAK Chapter 8; POH Section 7

7. Question 7 · PA.I.G

   During run-up, you switch to Left mag and RPM doesn't drop at all. Is this good or bad?

   What a DPE expects to hear

   • Bad — indicates hot mag, P-lead likely detached
   • Engine won't shut off when key turned to OFF
   • Ground safety hazard near prop

   SourcePHAK Chapter 7; POH Section 4

8. Question 8 · PA.I.G

   Oil temperature is rising rapidly in a climb. What should you do and why does oil matter for cooling?

   What a DPE expects to hear

   • Oil provides lubrication AND cooling for internal engine parts
   • Action: level off to increase airflow and/or enrich mixture

   SourcePHAK Chapter 7; POH Section 3