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FULL INSTRUMENTS

Altitude

Pressure vs Altitude

This instrument uses an aneroid capsule assembly to measure the atmospheric pressure outside of an aircraft.

The pressure at a given altitude is a function of the air supported above that point where the force is caused by gravity. As the column of air is taller closer to earth, there is more pressure. However, this is not a linear progression due to the fact that air is compressible.

 

 

Altitude Pressure Chart

The Altitude Pressure Chart provides a visual representation of what the instrument must be calibrated to measure. There are only very slight changes in pressure past 30,000 feet.

The nature of an altitude instrument requires a great deal of sensitivity as the atmospheric air pressure approaches a vacuum.

 

Aneroid Capsule

The aneroid capsule assembly has a deep vacuum trapped inside, and measures the difference between the atmosphere outside of the aircraft, and this nearly perfect vacuum. The internal vacuum is constant and provides the reference pressure that the capsule uses to measure altitude.

Temperature Effect

An important consideration is temperature. Heat reduces the density of air resulting in a lower pressure due to the force of gravity. To know the true altitude, both the atmospheric pressure and the ambient temperature would be required. This class of altitude instrument is not temperature compensated.

This does not invalidate the usefulness of this instrument - the temperature of the atmosphere gets colder, and is more predictable as altitude increases above 10,000 feet. Operating from 20,000 to 50,000, the instrument will be accurate.

As the aircraft approaches an airport the controller will advise the "temperature altitude" at the runway. In Denver during the summer, this value can be as high as 9,000 feet. The pilot knows that when his altitude instrument indicates 11,000 feet, the aircraft is really 2,000 feet above the ground.

 

Instrument Application

This type of altitude instrument is not usually used for primary flight in newer general aviation. Contemporary microprocessor based altimeters, and radar altimeters coupled with heads up and in dash displays provide for more accuracy without other considerations on the part of the pilot.

These instruments remain in the aircraft for emergency use, and in the hands of a skilled pilot will still allow for a safe landing.

Other applications include balloons and ultra-light aircraft where un-powered mechanical instruments would be attractive.

Airspeed

Differential Pressure vs Airspeed

This instrument uses a capsule assembly to measure the differential pressure produced by a pitot static tube mounted outside of the aircraft.

The pressure at a given airspeed is a function of the impact pressure of the air hitting the aircraft, and the ambient pressure surrounding the aircraft. However, this is not a linear progression due to the fact that impact force is a function of kinetic energy which has a square function.

 

Airspeed Pressure Chart

The Airspeed Pressure Chart provides a visual representation of what the instrument must be calibrated to measure. There are only very slight changes in pressure below 50 knots. The nature of an airspeed instrument requires a great deal of sensitivity at low speeds.

Differential Capsule

The differential capsule assembly measures the difference between the impact pressure at the tip of a pitot tube, and the atmospheric pressure outside of the aircraft. If the aircraft is stationary with no impact pressure, the differential capsule will indicate zero at any altitude - this is because the ambient pressure is applied equally to both sides of the sensor.

Altitude and Temperature Effect

A consideration is altitude and temperature. Both heat and altitude reduce the density of air resulting in a lower impact pressure, and a reduced indicated airspeed. This class of altitude instrument is not temperature or altitude compensated.

This does not reduce the usefulness of this instrument. Because the wing requires more airspeed to generate lift in less dense air, the indicated airspeed is very important - the plane will stall at the same indicated airspeed at any temperature altitude. At low altitudes during takeoff and landing, the errors are small.

Instrument Application

This type of airspeed instrument is not usually used for primary flight in newer general aviation. Contemporary microprocessor based air data computers, and radar sensors coupled with heads up and in dash displays, provide far more accuracy without other considerations on the part of the pilot.

These instruments remain in the aircraft for emergency use, and in the hands of a skilled pilot will still allow for a safe landing.

Other applications include balloons and ultra-light aircraft where un-powered mechanical instruments would be attractive.

 

Vertical Speed

Differential Pressure Generated by Time Delay

This instrument uses a capsule assembly to measure the differential pressure produced by changes in altitude. The differential pressure is created by measuring two volumes - one attached directly the atmosphere outside the aircraft, and the other attached via a restrictor. The restrictor makes the second volume respond slowly to changes in pressure, which are the result of changes in altitude. This time lag is directly related to the altitude rate of change.

Differential Capsule 

The differential capsule assembly measures the difference between the two volumes. If the aircraft is in level flight the differential capsule will indicate zero at any altitude - this is because the ambient pressure is applied equally to both sides of the sensor and the slow volume has equalized.

The pressure difference between the two volumes is very small, and the capsule requires high sensitivity, especially at the low end of the scale.

Lag Error

The vertical speed measurement is based upon the slow response time of one volume in the instrument, and by nature requires about 6 seconds to stabilize after a change in rate.

This does not affect the usefulness of this instrument. Most of the time an aircraft is either level, or in a constant rate climb or decent. Monitoring constant rate is the primary use for the indicator. The pilot is aware during rapid maneuvers that the vertical speed indicator is not operating in real time.

Instrument Application

This type of vertical speed instrument is not usually used for primary flight in newer general aviation. Contemporary microprocessor based altimeters, and radar altimeters coupled with heads up and in dash displays, provide far more accuracy without other considerations on the part of the pilot.
These instruments remain in the aircraft for emergency use, and in the hands of a skilled pilot will still allow for a safe landing.

Other applications include balloons and ultra-light aircraft where un-powered mechanical instruments would be attractive.

Instrument Application

This type of vertical speed instrument is not usually used for primary flight in newer general aviation. Contemporary microprocessor based altimeters, and radar altimeters coupled with heads up and in dash displays, provide far more accuracy without other considerations on the part of the pilot.
These instruments remain in the aircraft for emergency use, and in the hands of a skilled pilot will still allow for a safe landing.

Other applications include balloons and ultra-light aircraft where un-powered mechanical instruments would be attractive.

 

Altitude Pressure Chart

The Altitude Pressure Chart provides a visual representation of what the instrument must be calibrated to measure. There are only very slight changes in pressure past 30,000 feet.

The nature of an altitude instrument requires a great deal of sensitivity as the atmospheric air pressure approaches a vacuum.

 

Aneroid Capsule

The aneroid capsule assembly has a deep vacuum trapped inside and measures the difference between the atmosphere outside of the aircraft and this nearly perfect vacuum. The internal vacuum is constant and provides the reference pressure that the capsule uses to measure altitude.

Temperature Effect

An important consideration is temperature. Heat reduces the density of air resulting in a lower pressure due to the force of gravity. To know the true altitude, both the atmospheric pressure and the ambient temperature would be required. This class of Cabin Pressure instrument is not temperature compensated.

Instrument Application

This type of Cabin Pressure instrument is not always used for primary flight in newer general aviation. Contemporary microprocessor based transducers are coupled with heads up and in dash displays in the modern glass cockpit.

These instruments also remain in the aircraft for emergency use, and provide an alternative monitor of this life critical parameter.

 

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