Air Temperature MeasurementNavigation — Instrumentation — DGCA CPL practice questions
Question 1 of 8
The difference between static air temperature and total air temperature is known as:
All 8 questions — Air Temperature Measurement
Navigation — Instrumentation · DGCA CPL. The correct option is marked on each.
Q1. The difference between static air temperature and total air temperature is known as:
- A.Corrected outside air temperature
- B.The total ram rise✓
- C.The recovery factor
- D.Hot ramp radiation
Why: Total Ram Rise = TAT − SAT. It is the theoretical maximum temperature increase caused by adiabatic compression when the moving air is brought to rest. See Section 5 . — Memorise: TAT − SAT = Total Ram Rise. RAT − SAT = Measured Ram Rise. K r links them: Measured = Total × K r .
Q2. A direct reading aircraft thermometer usually consists of a bimetallic helix protruding into the airstream. Movement of the pointer over the temperature scale will depend upon:
- A.Difference in electrical resistance of the two metals
- B.Increase in pressure as airspeed increases
- C.Increase in adiabatic cooling as airspeed increases
- D.Different coefficients of expansion of the two metals✓
Why: The bimetallic strip works because brass and invar have different coefficients of thermal expansion. On heating, brass expands more than invar, causing the strip (wound as a helix) to curl and move the pointer. See Section 2 . — Direct Reading = bimetallic (expansion principle). Remote Reading = resistance wire (electrical principle). These are the two fundamental operating principles.
Q3. A remote reading thermometer depends on ………. to indicate changes in temperature:
- A.Change of electrical resistance of the two metals
- B.Change of electrical resistance with temperature✓
- C.Change of electrical resistance with pressure
- D.Change of electrical capacitance with temperature
Why: The TAT probe uses a pure platinum resistance wire. Platinum has a precise and predictable resistance-temperature relationship: resistance increases as temperature rises. This change is measured and converted to a temperature readout. See Section 3 . — Pure platinum is used because of its exceptionally linear and repeatable resistance-temperature characteristic (positive temperature coefficient).
Q4. Aircraft air temperature thermometers are shielded to protect them from:
- A.Solar radiation✓
- B.Accidental physical damage on the ground or hailstones in flight
- C.Airframe icing
- D.Kinetic heating
Why: The probe strut is designed to shield the sensing element from direct solar radiation, which would heat the element above the true air temperature and cause an over-reading error. See Section 4 . — Error → Remedy mapping: Solar radiation → shielding; Ice accretion → heater; Kinetic/adiabatic heating → Recovery Factor correction.
Q5. At a true airspeed of 500 knots, what is the ram rise?
- A.50°C
- B.25°C✓
- C.5°C
- D.16°C
Why: Using the rapid formula: Ram Rise = (TAS/100)² = (500/100)² = 5² = 25°C . See Section 7 . — Quick table: 200 kt = 4°C; 300 kt = 9°C; 400 kt = 16°C; 500 kt = 25°C. (All from squaring TAS/100.)
Q6. An air temperature probe may be aspirated in order to:
- A.Prevent icing
- B.Measure air temperature on the ground✓
- C.Compensate for moisture level at the ramp position
- D.Reduce the effects of solar radiation
Why: When the aircraft is stationary, there is no natural airflow through the probe. An aspirator uses engine or APU bleed air to draw fresh outside air through the casing, preventing heat-soaked stagnant ramp air from contaminating the temperature reading. This is essential for accurate take-off performance calculations. See Section 3 . — Aspirator = ground-use device. It uses bleed air suction to replace stagnant hot ramp air with fresh outside air at the sensing element.
Q7. Total Air Temperature is:
- A.The maximum temperature attainable by the air when brought to rest adiabatically✓
- B.The temperature indicated on the air temperature thermometer plus the ram rise
- C.The static air temperature minus the recovery factor
- D.The recovery factor plus the ram rise
Why: This is the precise definition of TAT — the theoretical upper limit when all kinetic energy of the moving air is converted to heat through adiabatic compression, with no losses. See Section 5 . — Learn the definition verbatim: "Total Air Temperature is the maximum temperature attainable by the air when brought to rest adiabatically." This exact phrasing appears in exams.
Q8. Which of these formulae gives the total temperature (T t ) from the static temperature (T s ):
- A.T t = T s (1 + 0.2 M²)
- B.T t = T s (1 + 0.2 K r M²)✓
- C.T t = T s / (1 + 0.2 K r M²)
- D.T t = T s (1 − 0.2 M²)
Why: The accurate formula SAT = TAT / (1 + 0.2 K r M²) rearranges to TAT = SAT × (1 + 0.2 K r M²). This includes Recovery Factor K r , which accounts for probe inefficiency. All temperatures must be in Kelvin. See Section 8 . — Direction memory aid: TAT is always GREATER than SAT — so going SAT→TAT, multiply by a factor >1. Going TAT→SAT, divide by that same factor.