ATPL Instruments Questions

Approximately what is the speed of sound at sea level in the International Standard Atmosphere?

• 661 kt
• 343 kt
• 1,000 kt
• 500 kt

Explanation: In the ISA at sea level, the local speed of sound is approximately 661 knots (about 340 m/s). Since the speed of sound depends on air temperature, this value decreases as temperature falls with altitude, up to the tropopause.

Instruments - Speed of Sound & Mach Number

At sea level in the International Standard Atmosphere, how does calibrated airspeed (CAS) compare to true airspeed (TAS)?

• CAS equals TAS
• CAS is always greater than TAS
• CAS is always less than TAS
• There is no relationship between them at sea level

Explanation: At sea level under ISA conditions (standard temperature and pressure, and therefore standard density), calibrated airspeed and true airspeed are equal. As altitude increases and air density decreases, TAS becomes progressively greater than CAS for a given dynamic pressure.

Instruments - Airspeed Definitions

A traditional gyroscopic heading indicator (directional gyro) is subject to a slow drift error over time, generally caused by:

• Real-world imperfections (friction and the earth's rotation) causing gradual precession, requiring periodic realignment against the magnetic compass
• A deliberate design feature intended to correct for magnetic variation automatically
• Direct interference from nearby VOR ground stations
• Rapid changes in outside air temperature alone

Explanation: A basic directional gyro isn't slaved to the magnetic compass and gradually drifts due to small real-world imperfections such as gyro friction and apparent drift caused by the earth's own rotation, so it needs to be periodically checked and realigned against the magnetic compass to stay accurate.

Instruments - Heading Indicator Errors & Realignment

What is the general purpose of the tilt control on an airborne weather radar display?

• To angle the radar beam up or down, letting the crew scan different altitudes of weather ahead and avoid excessive ground returns
• To adjust the color scheme used to depict precipitation intensity
• To switch between weather mode and ground-mapping mode only
• To change the radar's transmission frequency

Explanation: Tilt control lets the crew steer the radar beam's vertical angle, so they can scan the altitudes where significant weather is likely to be found ahead of the aircraft, while avoiding tilting the beam so low that ground returns clutter the display or so high that the radar overshoots the tops of nearby storms.

Instruments - Airborne Weather Radar Operation

Depending on engine design, which of the following might be used as the primary parameter for setting and monitoring thrust?

• Either N1 or engine pressure ratio (EPR), depending on the specific engine type
• Only cabin altitude, regardless of engine type
• Only outside air temperature, regardless of engine type
• Fuel tank quantity alone, on every engine type

Explanation: Different turbofan engine designs use different primary indications for setting and monitoring thrust - some are built around N1 (fan speed), while others use engine pressure ratio (EPR, comparing turbine discharge pressure to inlet pressure) - so the specific reference parameter a crew primarily monitors depends on the engine type installed.

Instruments - Engine Thrust-Setting Parameters

What does Mode S 'enhanced surveillance' generally add, beyond basic position and altitude reporting?

• The ability to transmit additional aircraft parameters (such as selected altitude or airspeed) to ground surveillance systems
• The ability to physically override another aircraft's flight controls
• A completely independent GPS position source
• Automatic weather radar data sharing between aircraft

Explanation: Mode S enhanced surveillance can transmit additional downlinked parameters - such as the crew's selected altitude, indicated airspeed, or heading - to ground-based surveillance systems, giving controllers extra situational information beyond the basic position and pressure-altitude reporting that a standard transponder provides.

Instruments - Mode S Enhanced Surveillance

What is the purpose of the 'sense' antenna used alongside a loop antenna in some ADF installations?

• To resolve the 180-degree ambiguity in the bearing indicated by the loop antenna alone
• To increase the overall range of the ADF receiver
• To provide a completely independent, second bearing source
• To filter out static from thunderstorm activity

Explanation: A simple loop antenna alone can indicate a bearing line to an NDB but can't distinguish which of the two opposite directions along that line the station actually lies in - combining the loop's signal with a sense antenna resolves this 180-degree ambiguity, giving a single, correct bearing rather than two possible ones.

Instruments - ADF Antenna Systems

How does ADS-B generally differ from TCAS as a means of traffic awareness?

• ADS-B broadcasts an aircraft's own precise position and other data for reception and cockpit display by others, rather than relying on interrogating nearby transponders as TCAS does
• ADS-B is simply an older name for the same TCAS system
• ADS-B can only be used on the ground and provides no airborne traffic information
• ADS-B requires no aircraft equipment at all to function

Explanation: TCAS actively interrogates the transponders of nearby aircraft and derives their position from the timing and bearing of the replies, whereas ADS-B has aircraft broadcast their own accurately known position (typically derived from satellite navigation) and other data, which can then be received and displayed directly by other aircraft or ground stations - generally providing more precise, more complete traffic information than an interrogation-based system.

Instruments - ADS-B Traffic Awareness