Can someone tell me how pressure controller allows for varying airfield altitudes.?

Question

Can someone tell me how pressure controller allows for varying airfield altitudes.?

Answer 1

To understand you have to know a little about aircraft instruments.  This is a bit long, but should answer your question.

A typical instrument can be compared to a clock, in that the instrument has a mechanism, or works; a dial, or face; pointers, or hands; and a cover glass.  The instrument mechanism is protected by a one; or two-piece case.  Various materials, such as aluminum alloy, magnesium alloy, iron, steel, or plastic are used in the manufacture of instrument cases.  Bakelite is the most commonly used plastic.  Cases for electrically operated instruments are made of iron or steel; these materials provide a path for stray magnetic force fields that would otherwise interfere with radio and electronic devices.

Some instrument mechanisms are housed in airtight cases, while other cases have a vent hole. The vent allows air pressure inside the instrument ease to vary with the aircraft's change in attitude.

The altimeter in the aircraft is a barometric instrument.  With the aircraft on the ground, the altimeter hands should be set to zero and the instrument panel tapped lightly a few times to remove any possible frictional errors.  The barometer scale on the altimeter face will indicate local atmospheric pressure when the altimeter hands are at zero.

Three of the most important flight instruments are connected into a pitot-static system.  These instruments are the airspeed indicator, the altimeter, and the rate-of-climb indicator.

There are many kinds of altimeters in general use today.  However, they are all constructed on the same basic principle as an aneroid barometer.  They all have pressure-responsive elements (aneroids) which expand or contract with the pressure change of different flight levels.  The heart of an altimeter is its aneroid mechanism.  The expansion or contraction of the aneroid with pressure changes actuates the linkage, and the indicating hands show altitude.  Around the aneroid mechanism of most altimeters is a device called the bimetal yoke.  As the name implies, this device is composed of two metals and performs the function of compensating for the effect that temperature has on the metals of the aneroid mechanism.

The presentation of altitude by altimeters in current use varies from the multi-pointer type to the drum and single pointer, and the digital counter and single pointer types.

The dial face of the typical altimeter is graduated with numerals from zero to 9 inclusive.  Movement of the aneroid element is transmitted through a gear train to the three hands on the instrument face.  These hands sweep the calibrated dial to indicate the altitude of the aircraft.  The shortest hand indicates altitude in tens of thousands of feet; the intermediate hand, in thousands of feet; and the longest hand, in hundreds of feet in 20-ft. increments. A barometric scale, located at the right of the instrument face, can be set by a k**b located at the lower left of the instrument case.  The barometric scale indicates barometric pressure in inches of mercury.

Since atmospheric pressure continually changes, the barometric scale must be re-set to the local station altimeter setting before the altimeter will indicate the correct altitude of the aircraft above sea level.  When the setting k**b is turned, the barometric scale, the hands, and the aneroid element move to align the instrument mechanism with the new altimeter setting.

Two setting marks, inner and outer, indicate barometric pressure in feet of altitude.  They operate in conjunction with the barometric scale, and indications are read on the altimeter dial.  The outer mark indicates hundreds of feet, and the inner mark thousands of feet.  Since there is a limit to the graduations which can be placed on the barometric scale, the setting marks are used when the barometric pressure to be read is outside the limits of the scale.

Answer 2

Sure. The field elevation is dialed in prior to decent so that the system can bring the cabin altitude down at an easy rate, about 300 ft/min is good, so that it equals the field elevation at about the time the aircraft does. You would not want to have the cabin descend to sea level for instance and then have to climb back up a mile so the pressure will be equal inside and outside the cabin after landing at Denver.