022 Instrumentation

A phonic wheel rotates at the engine's RPM. Next to the wheel a speed probe with a permanent magnet is installed. The magnetic field of the magnet is disturbed by the rotating wheel. Therefore, the magnetic field depends on the rotating speed of the wheel and hence can be translated into a RPM. --> Requires electrical power from an aircraft source

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For multi-engine ACFT:

• Avoid discomfort and fatigue from noise
• Detects and indicates difference in RPM
• Uses the engine tachometer (RPM)
• Does not need electrical power

--> Ensures that all engines run synchronously

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Commercial ACFT

• Normal time indication
• GPS based
• Used for ACARS

Small ACFT

• Normal time indication
• HOBBS or Flight time measurement
  Variant 1: Connected to the battery. Measures time as of which the battery is used until shut down
  Variant 2: Connected to the engine. Measure time as of which the engines runs until shut down
  Variant 3: Starts and stops at defined airspeed (50 to 60kt)
• Used for maintenance

Measure static pressure P_S

One on each side of the plane --> Increases accuracy

Ports are subject to Position error influenced by:

• Pitch attitude
• Speed changes
• Configuration changes

Measures total pressure P_T

One attached at the front of the wing to keep it in undisturbed air flow.

Subject to total pressure error if pitch > 20° up or down

Capsule: Very low pressure (vacuum)

Case: Static pressure

Contains a bimetal temperature compensator

Tolerances:

• up to 30'000ft +/- 60ft
• up to 50'000ft +/- 80ft

1. Simple altimeter
   One aneroid capsule
2. Sensitive altimeter
   Multiple aneroid capsule enhance accuracy
3. Servo assisted altimeter
   Pressure measurement is connected to a circuit, translating contraction/expansion of capsules into electric signals. --> Eliminates the lag error and makes it the most accurate altimeter

• Position error
• Instrument error
• Barometric error
• Temperature error
• Lag error

Capsule: Static pressure

Case: Static pressure

Static pressure is directly fed to the capsule while the case has a restricted passage through which static pressure enters. --> The instrument measures the difference between static pressure in the capsule and case.
Once leveled-off the pressure in the case reaches the static pressure in the capsule and the VSI indicates 0 again.

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Static system errors

• Blocked static port
• Leakage of static line
• Position error

Instrumentation error

Time lag

Working principle is the same as for the regular VSI but an acceleration pump is added which helps to pass the pressure quicker and hence reduces the time lag of the indication?

Case: Static pressure

Capsule: Total pressure

...

V_S0 = Stalling speed in landing configuration
V_S1 = Stalling speed in a specified configuration
V_FE = Max. speed with flaps full down
V_LE = _{Max. landing gear extended speed}
V_LO = _{Max. landing gear operating speed}
--> V_S0 bis V_FE = White sector

V_NO = Max. Normal operating speed
--> V_FE bis V_NO = Green sector (normal operating range)

V_NE = Never exceed speed
--> V_NO bis V_NE = Yellow sector (caution range)

V_X = Best rate of climb
V_Y = Best angle of climb
V_MC = Minimum control speed, one engine inoperative, take-off configuartion, landing gear retracted, running engine at T/O power, max. 5° bank.

Total Air Temperature

Measured by the Instrument 

TAT = SAT + Ram Rise --> Always higher than SAT

Static Air Temperature

Actual Temperature

Lower than TA

Is the difference between TAT and SAT

Induced by friction of the air at the sensor --> varies with speed (occurs above 180kt/ M 0.2)

\(RR = SAT * 0.2 * M^2\)

Pitot tube measures total Pressure P_T. Static ports measure static pressure P_S.
If static ports are blocked alternate static source can be opened in the cockpit (P_S,ports > P_S,alternate !).

Chamber of ASI is fed with P_T. Aneroid capsule is fed with P_S --> P_D is calculated and shown on ASI (P_T - P_S = P_D)

Indicated Airspeed (IAS)

Calibrated Airspeed (CAS)

• Position Error
• Instrument Error

Equivalent Airspeed (EAS)

• Compressibility

Trues Airspeed (TAS)

• Density Error

Stall speed landing configuration

Stall Speed specified configuration

Max. Flaps extended speed

Normal Operating limit speed

Never Exceed speed

Maximum landing gear operation speed

Maximum landing gear extended speed 

• Static Pressure in Case
• Very low pressure/vacuum in capsule
  • Capsules are pressed or expand and thus indicate the altitude 
• Error: Blocked Static Ports: Alt stays 

Vertical Speed indicator

• Static Pressure in Capsule, direct static
• Static pressure in case, delayed static
• Difference between direct and delayed static pressure gives the vertical speed
• Error: Blocked static port: shows zero