Operation Of Systems - C: Pitot/Static Flight Instruments Flashcards
Which instruments operate off of the pitot/static system?
Altimeter, vertical speed, and airspeed indicator.
How does an altimeter work?
A sensitive altimeter is an aneroid barometer that measures the absolute pressure of the ambient air and displays it in terms of feet above a selected pressure level. The sensitive element in a sensitive altimeter is a stack of evacuated, corrugated bronze aneroid capsules. The air pressure acting on these aneroids tries to compress them against their natural springiness, which tries to expand them. The result is that their thickness changes as the air pressure changes. Stacking several aneroids increases the dimension change as the pressure varies over the usable range of the instrument.
What are the limitations of a pressure altimeter?
Nonstandard pressure and temperature; temperature variations expand or contract the atmosphere and raise or lower pressure levels that the altimeter senses.
On a warm day—The pressure level is higher than on a standard day. The altimeter indicates lower than actual altitude.
On a cold day—The pressure level is lower than on a standard day. The altimeter indicates higher than actual altitude.
Changes in surface pressure also affect pressure levels at altitude.
Higher than standard pressure—The pressure level is higher than on a standard day. The altimeter indicates lower than actual altitude.
Lower than standard pressure—The pressure level is lower than on a standard day. The altimeter indicates higher than actual altitude.
Remember: High to low or hot to cold, look out below!
Define and state how you would determine the following altitudes: absolute, indicated, pressure, true, and density.
Absolute altitude—the vertical distance of an aircraft above the terrain.
Indicated altitude—the altitude read directly from the altimeter (uncorrected) after it is set to the current altimeter setting.
Pressure altitude—the altitude when the altimeter setting window is adjusted to 29.92. Pressure altitude is used for computer solutions to determine density altitude, true altitude, true airspeed, etc.
True altitude—the true vertical distance of the aircraft above sea level. Airport, terrain, and obstacle elevations found on aeronautical charts are true altitudes.
Density altitude—pressure altitude corrected for nonstandard temperature variations. Directly related to an aircraft’s takeoff, climb, and landing performance.
How does the airspeed indicator operate?
The airspeed indicator is a sensitive, differential pressure gauge which measures the difference between impact pressure from the pitot head and undisturbed atmospheric pressure from the static source. The difference is registered by the airspeed pointer on the face of the instrument.
What is the limitation of the airspeed indicator?
The airspeed indicator is subject to proper flow of air in the pitot/static system.
What are the errors of the airspeed indicator?
Position error—Caused by the static ports sensing erroneous static pressure; slipstream flow causes disturbances at the static port preventing actual atmospheric pressure measurement. It varies with airspeed, altitude and configuration, and may be a plus or minus value.
Density error—Changes in altitude and temperature are not compensated for by the instrument.
Compressibility error—Caused by the packing of air into the pitot tube at high airspeeds, resulting in higher than normal indications. It is usually not a factor at slower speeds.
What are the different types of aircraft speeds?
Indicated airspeed (IAS)—the speed of the airplane as observed on the
airspeed indicator. It is the airspeed without correction for indicator,
position (or installation), or compressibility errors.
Calibrated airspeed (CAS)—the airspeed indicator reading corrected for
position (or installation), and instrument errors. CAS is equal to TAS at
sea level in standard atmosphere. The color-coding for various design
speeds marked on airspeed indicators may be IAS or CAS.
Equivalent airspeed (EAS)—the airspeed indicator reading corrected for
position (or installation), or instrument error, and for adiabatic
compressible flow for the particular altitude. EAS is equal to CAS at sea
level in standard atmosphere.
True airspeed (TAS)—CAS corrected for altitude and nonstandard
temperature; the speed of the airplane in relation to the air mass in which
it is flying.
Name several important airspeed limitations not marked on the face of the airspeed indicator.
Design maneuvering speed (Va)—the maximum speed at which the structural design’s limit load can be imposed (either by gusts or full deflection of the control surfaces) without causing structural damage.
Landing gear operating speed (Vlo)—the maximum speed for extending or retracting the landing gear if using aircraft equipped with retractable landing gear.
Best angle-of-climb speed (Vx)—important when a short-field takeoff to clear an obstacle is required.
Best rate-of-climb speed (Vy)—the airspeed that will give the pilot the most altitude in a given period of time.
What airspeed limitations apply to the color-coded marking system of the airspeed indicator?
White Arc flap operating range
Lower A/S Limit
White Arc V S0 (stall speed landing configuration)
Upper A/S Limit
White Arc V FE (maximum flap extension speed)
Green Arc normal operating range
Lower A/S Limit
Green Arc V S1 (stall speed clean or specified configuration)
Upper A/S Limit
Green Arc V NO (normal operations speed or maximum
structural cruise speed)
Yellow Arc Caution Range (operations in smooth air only)
Red Line V NE (never exceed speed; above this speed,
structural failure may occur.)
How does the vertical speed indicator work?
The vertical speed indicator is a pressure differential instrument. Inside the instrument case is an aneroid very much like the one in an airspeed indicator. Both the inside of this aneroid and the inside of the instrument case are vented to the static system, but the case is vented through a calibrated orifice that causes the pressure inside the case to change more slowly than the pressure inside the aneroid. As the aircraft ascends, the static pressure becomes lower and the pressure inside the case compresses the aneroid, moving the pointer upward, showing a climb and indicating the number of feet per minute the aircraft is ascending.
What are the limitations of the vertical speed indicator?
The VSI is not accurate until the aircraft is stabilized. Because of the restriction in airflow to the static line, a 6 to 9 second lag is required to equalize or stabilize the pressures. Sudden or abrupt changes in aircraft
attitude will cause erroneous instrument readings as airflow fluctuates over the static port. Both rough control technique and turbulent air result
in unreliable needle indications.
Name the three types of error to which Pitot-static systems are susceptible.
- Position Error - caused by airflow past the static port at a different speed than AC TAS - AoA, Configuration, weight, Airspeed
- Fixed Position Error: specific to the design of that AC make/model
- Variable Position Error: deformed panels, obstruction, slips, skids etc. - Lag Errors: time it takes for the change to get to the instrument
- Happens at the instant of change, then stabilizes. i.e VSI,
Airspeed indicator symptoms of Clogged Static port
Airspeed freezes in straight and level flight
Shows lower speed at higher elevation
Shows higher speed at Lower elevation
Airspeed indicator Symptoms of clogged Pitot tube, and blocked drain hole
Airspeed freezes - Becomes an altimeter increasing as you climb from elevation of blockage, decreasing as you descend
