SAMRA/SARON Instruments Questions

If an aircraft's pitot tube becomes completely blocked by ice while the drain hole remains clear, and the aircraft then climbs, the airspeed indicator will

• behave like an altimeter, showing an increasing indicated airspeed as the aircraft climbs, unrelated to the aircraft's actual speed.
• show a decreasing indicated airspeed as the aircraft climbs, the reverse of the actual pitot-blockage effect described above.
• continue to read accurately throughout the climb, since the open drain hole is assumed to fully compensate for the blocked pitot line.
• immediately drop to zero and remain there for the rest of the flight, since no pitot pressure at all can reach the diaphragm.

Explanation: With the pitot line sealed but the static line and the drain hole still open to ambient pressure, the trapped pitot pressure stays fixed at the value it had when the blockage occurred, while the static pressure in the case continues to fall as the aircraft climbs. Since airspeed is derived from the difference between those two pressures, a falling static pressure against a fixed trapped pitot pressure makes the instrument read an increasing speed -- it starts behaving like an altimeter rather than an airspeed indicator.

TP 690 Section 3 - Pitot Static System

A torquemeter is used mainly on turboprop engines because

• turboprop engines have no tachometer at all installed, so torque is the only rotational parameter ever made available.
• since a constant-speed propeller holds RPM near-constant, RPM alone does not indicate the power being delivered.
• torquemeters are required only during engine start, and are disregarded for the remainder of the flight.
• torque directly measures fuel flow, making a separate dedicated fuel flow gauge entirely unnecessary.

Explanation: Because a constant-speed propeller keeps RPM close to a fixed value across a broad power range, RPM stops being a useful indicator of power output on a turboprop. Torque -- the twisting force actually delivered to the propeller shaft -- is what varies with power, which is why turboprops rely on a torquemeter (often expressed as a percentage or in foot-pounds) as their primary power-setting parameter, alongside the tachometer.

TP 690 Section 3 - Torquemeter

If a pitot tube becomes blocked but its drain hole remains clear, the airspeed indicator will subsequently tend to behave like

• a vertical speed indicator, displaying a rate of change rather than any absolute value at all, despite the ASI's underlying mechanism having no relationship whatsoever to rate-of-change sensing.
• a perfectly accurate airspeed indicator, since a clear drain hole is sufficient to maintain fully normal ASI function despite the blockage.
• an altimeter, since trapped pitot pressure stays roughly constant while static pressure continues to vary with altitude, so the ASI reads higher climbing and lower descending.
• a completely frozen, unmoving instrument, exactly as it would if the drain hole were also blocked.

Explanation: With the pitot inlet blocked but the drain hole open, trapped pitot pressure stays essentially fixed while static pressure -- effectively still able to act on the system through the drain hole path -- keeps changing with altitude. The ASI, which normally displays the difference between pitot and static pressure, ends up behaving like an altimeter instead: it reads higher than actual while climbing and lower than actual while descending, a genuinely different (and importantly, different-to-recognize) failure mode than a fully blocked pitot system with no drain hole path at all.

TP 690 Section 3 - Instruments, Pitot-Static System

On a piston engine, Exhaust Gas Temperature (EGT) and Cylinder Head Temperature (CHT) gauges are commonly used by a pilot primarily to

• replace the need for a tachometer entirely, since EGT and CHT are considered equivalent substitutes for monitoring engine RPM.
• measure the aircraft's outside air temperature at cruising altitude, a purely meteorological rather than an engine-management function, a description inconsistent with EGT and CHT's actual role in mixture leaning and cylinder temperature monitoring.
• indicate the temperature of the aircraft's hydraulic fluid, an unrelated system with no connection to piston engine mixture or cylinder condition.
• help set and monitor mixture (leaning) and engine operating temperature, guarding against operating conditions that could cause detonation or other engine damage.

Explanation: EGT and CHT give the pilot practical, real-time feedback for mixture leaning (EGT typically referenced against a peak/rich-of-peak or lean-of-peak leaning technique) and for monitoring that cylinder head temperature stays within a safe operating range, helping avoid conditions like detonation or excessive thermal stress that could damage the engine -- a piston-engine analog to the turbine ITT/TIT temperature monitoring covered elsewhere, but for a different engine type and mechanism, and unrelated to outside air temperature, hydraulic fluid temperature, or engine RPM (which a tachometer measures separately).

TP 690 Section 3 - Instruments

True airspeed (TAS) is derived from calibrated airspeed (CAS) by applying a correction for

• runway slope, a factor relevant only to takeoff and landing performance calculations on the ground, rather than to the derivation of true airspeed from calibrated airspeed while the aircraft is airborne in flight.
• air density, which is determined from pressure altitude and outside air temperature, since CAS reads increasingly low relative to true airspeed as air density decreases with altitude.
• magnetic variation, since TAS is fundamentally a heading-related correction rather than a speed correction based on air density.
• compass deviation, a correction that applies to heading indication rather than to the relationship between airspeed values.

Explanation: True airspeed is obtained by correcting calibrated airspeed for air density, which is determined from pressure altitude and outside air temperature -- as air density decreases with increasing altitude, CAS reads increasingly low relative to TAS, requiring this correction (commonly a function performed automatically by an air data computer or Machmeter/TAS gauge); magnetic variation, compass deviation, and runway slope are unrelated corrections used for entirely different purposes.

TC AIM AIR - Airspeed Definitions

If an aircraft's pitot tube becomes completely blocked AND its drain hole also becomes blocked, trapping a fixed quantity of air in the pitot line, the airspeed indicator will subsequently tend to behave, during a climb, like

• an indicator that immediately and permanently reads zero the instant both the pitot tube and its drain hole become blocked.
• a vertical speed indicator, displaying the aircraft's rate of climb rather than any airspeed-like value.
• an altimeter, with the indication generally increasing with altitude as the outside static pressure surrounding the trapped, now-fixed pitot pressure continues to fall.
• an accurate airspeed indicator throughout the climb, since a blocked drain hole is defined as having no additional effect beyond the pitot blockage alone.

Explanation: With both the pitot tube and its drain hole blocked, the pressure trapped in the pitot line becomes fixed at whatever it was at the moment of blockage, while the static pressure side continues to respond normally to changing altitude; because the ASI reads based on the difference between pitot and static pressure, a climb (falling static pressure) causes the indicated airspeed to rise even though true airspeed may not have changed, making the instrument behave much like an altimeter for the remainder of the flight -- a different (and in some ways more insidious) failure mode than a blocked pitot tube with its drain hole still clear.

TP 690 -- Instruments: Pitot-Static System

An autopilot disconnect warning, generally consisting of both an aural tone and a flashing visual annunciation, is provided primarily so that

• the crew can silence all further aural alerts on the flight deck for the remainder of the flight.
• the autopilot automatically re-engages itself a few seconds after any disconnect, without requiring crew input.
• it applies only when the autopilot is disconnected manually by the crew, remaining silent for any automatic, uncommanded disconnect, which gets the priority backwards, since the aural warning is arguably most important for an unexpected, automatic disconnect that the crew has not commanded and might otherwise fail to notice promptly.
• the crew is immediately and unambiguously alerted whenever the autopilot disconnects, whether through deliberate crew action or an unexpected automatic disconnect, preventing the aircraft from being left temporarily unflown if the disconnect goes unnoticed.

Explanation: An autopilot disconnect warning -- typically a distinct aural tone paired with a flashing annunciation -- alerts the crew immediately whenever the autopilot disconnects, whether the crew deliberately disconnected it or the autopilot disconnected itself automatically in response to a fault or an out-of-limits condition; this is particularly important because an unnoticed disconnect could otherwise leave the aircraft briefly unflown by either the automation or a hand-flying pilot, at a moment the crew may not expect.

TP 690 -- Instruments: Autopilot Systems

A groundspeed readout, typically displayed on the Navigation Display or Flight Management System, is distinguished from true airspeed (TAS) in that groundspeed

• represents the aircraft's actual speed over the ground, incorporating the effect of wind, whereas TAS represents speed relative to the surrounding air mass alone
• is always identical to TAS, since both are computed from the same air data inputs, regardless of aircraft type or avionics suite
• can only be displayed when the aircraft is below 10,000 feet, a characterization that does not match how this instrument or system actually functions
• is derived exclusively from the pitot-static system, with no reference to navigation sensors, regardless of aircraft type or avionics suite

Explanation: Groundspeed reflects the aircraft's actual speed over the ground, incorporating the effect of wind, and is generally derived from navigation sensors such as GNSS or an inertial system, whereas TAS reflects speed relative to the surrounding air mass alone, independent of wind.

TP 690 -- Instruments: Groundspeed versus true airspeed display