Pilot Training: Flight Instruments Primer

PITOT-STATIC SYSTEM

The readings of the airspeed indicator, altimeter and rate-of-climb indicator depend upon two different pressures. One is called pitot (impact or dynamic) and the other is called static (atmospheric). Actually the only instrument affected by both types of pressure is the airspeed indicator. The altimeter and rate-ofclimb indicator depend solely upon static pressure.

Pitot-Static Head

A pitot-static head supplies both pitot and static pressures to the airspeed indicator and static pressure alone to the altimeter and rate-of-climb indicator.

The pitot pressure opening is on the forward end of the head, while the static pressure openings are located a short distance behind the forward head.

The head is so designed that any moisture or dirt entering it is trapped and kept from reaching the instruments themselves. A heating element is provided inside the head to prevent icing.

To deliver true pressure to the instruments the pitot-static head must be streamlined with the airflow. This can be accomplished usually only at cruising speed. Thus, inaccuracies may develop at other than cruising speed, but they are negligible except during takeoff, sharp pullouts, level-offs, and landings.

REMEMBER-Always make sure the pitot-static head cover is removed before takeoff. Don't leave the pitot heater turned on for extended periods on the ground. Except for testing purposes, turn it on only in flight when icing conditions are anticipated.

Flush Static Source

Static pressure is sometimes derived from a flush static pressure source, by means of a perforated flush-mounted plate, located usually well back on the airplane's fuselage. Pitot pressure in this system, however, is still obtained from a pitot head.

Alternate Static Source

Some airplanes have an alternate source of static pressure controlled by a switch or valve in the cockpit provided in case the usual source fails. For example, if the pitot-static head were broken off or the openings became iced, all three instruments in this group would become unreliable. By switching to the alternate you regain use of the rate-of-climb indicator and the altimeter but not the airspeed indicator, depending as it does on both pitot and static pressure.

The static pressure provided by the alternate source is usually lower than true or actual pressure. Thus, upon first switching to it, you find the rate-ofclimb indicator ordinarily indicating a climb, while the altimeter ordinarily indicates an increase in altitude, though none has occurred. The rate-ofclimb indicator returns to a normal indication shortly. The altimeter, however, continues to indicate an altitude other than actual.

Try the effect of the alternate source of your own airplane if so equipped, so you can be prepared for an emergency.

AIRSPEED INDICATOR

The actuating mechanism of the airspeed indicator is a metal diaphragm housed in an airtight case. Pitot pressure is admitted to the inside of the diaphragm. Static pressure is admitted to the case around the outside of the diaphragm. The difference between these two pressures causes the indications of this instrument and gives indicated airspeed (IAS). Any change in either pitot or STATIC  static pressure causes a change in the IAS.

You can estimate true airspeed (TAS) roughly by adding 2% of the IAS for each 1000 feet of altitude. For example, if the reading is 140 IAS at 20,000 feet of altitude, add 40% of 140 to 140, which gives an approximately correct reading of 196 TAS. For more accurate calculations use a computer.

REMEMBER-With the exception of your having wing ice or an increased load, the IAS at which your plane will stall is the same, no matter what the altitude or temperature. The airspeed indicator tells you accurately your safe airspeed limits, maximum or minimum. Make corrections for stalling speed if you have picked up wing ice or increased your normal load. See PIF 3-4-1 and High Speed Stalls, PIF 2-20-1.

ALTIMETER

The actuating mechanism of the altimeter is called an aneroid, which is a device for measuring absolute pressure. It is housed in an airtight case. Static pressure is admitted to the inside of the case, and acts on the aneroid so as either to compress it or allow it to expand. This compression or expansion causes movement of the hands on the face of the instrument.

The altimeter actually measures the weight of the air column (barometric pressure) above it and is so calibrated that a decrease in pressure causes an increase in its indication of altitude. The weight of the air column measured by the altimeter will vary because of changes in the air's density, which in turn is affected by variations in temperature. These factors often cause incorrect altimeter readings.

The altimeter is calibrated according to standard conditions of temperature and pressure, but these conditions seldom prevail. In order to correct for non-standard conditions modern altimeters are provided with a barometric scale.

Standard Altimeter Setting

Remember this system will always give you your height above sea-level, not above the terrain. Use these steps:

1. Before takeoff obtain the latest altimeter setting for the field from the tower. Then set the barometric scale on the instrument to this setting. The hands of the altimeter should then indicate the surveyed altitude of the field above sea-level. (If they do not, within the limits allowed on the scale-error card on your instrument panel, ask a mechanic to find the trouble. Often a small adjustment is enough to correct the error).

2. During flight you must continually correct the altimeter by resetting the barometric scale according to the latest altimeter setting of the area in which you are flying. Request it from the nearest radio facility.

3. Before landing again request the latest altimeter setting from the tower, and reset the instrument accordingly, so that you can depend on its accuracy.

RATE-OF-CLIMB INDICATOR

The rate-of-climb indicator has the same actuating mechanism as the airspeed indicator. A diffuser valve simply restricts the flow of the air from the case to the inside of the diaphragm. Only static pressure is admitted to the inside of the case and the outside of the diaphragm.

You get a reading when there is a difference in pressure between the inside and the outside of the diaphragm. The difference occurs chiefly in climbing or descending, when pressure inside and outside the diaphragm tend to equalize, but this takes some time because of the restriction in the flow of air between the diaphragm and case. Because of this restriction the instrument has a lag in its indications of several seconds.

Depend on the instrument only when establishing a constant rate of climb or descent in relatively smooth air. Use it in conjunction with the altimeter at all times as it indicates the rate of change and not the amount of change in altitude. The instrument is relatively useless in turbulence.

Effects of Pressure and Temperature on Altimeter

Standard atmospheric conditions at sea-level exist when the barometric pressure is 29.92 and the temperature is 15°C. The standard temperature for any altitude above sea-level decreases at the rate of 2°C for each 1000 feet of altitude. Remember these two rules:

ALTITUDE AND PRESSURE

Rule 1-in flying from an area of relatively high pressure into an area of lower pressure the actual altitude will become lower than the indicated altitude. This is a dangerous situation.

 ALTITUDE AND TEMPERATURE

Rule 2-When flying through air colder than standard the actual altitude becomes lower than the indicated altitude. WHAT TO DO: Calculate your true altitude by use of a computer.

WARNING

Before clearing for an instrument flight, be sure the altitude you have requested gives you sufficient terrain clearance over your entire route. Then keep your altimeter reading at this altitude. Don't make mental corrections and try to figure your true altitude. Keep your indicated altitude corrected by resetting your altimeter from settings given you over your radio check points.

MAGNETIC COMPASS

The magnetic compass is the basic navigational instrument. It consists of an airtight case filled with a special fluid in which a compass card assembly is pivoted. It is subject to many errors; mainly deviation and the errors produced by turning, accelerating or decelerating the airplane.

A compass correction card mounted on the instrument panel shows the amount of deviation.

Note the following errors in turning:

When turning toward North the indication of the compass lags considerably. In most sections of the United States this lag amounts to approximately 30'. When turning toward South the compass indication leads the airplane's direction on the heading by the same amount. These errors decrease as you make a turn toward East or West, becoming approximately zero on those headings.

The rule becomes, then; Overturn the heading when turning toward directions on the Southern half of the card (points between East and West through South). Underturn the heading when turning toward directions on the Northern half of the card (points between East and West through North)

.

When you accelerate or decelerate the airplane in an Easterly or Westerly direction you will notice an error as the compass turns off the direction in which you are flying.

When heading East or West acceleration causes the compass to indicate a turn toward the North. You get the same effect by going abruptly into a dive on these headings.

Deceleration on a heading of East or West causes the compass to indicate a turn toward the South. You get the same effect by going abruptly into a climb on these headings. These rules apply only in the Northern hemisphere. They are exactly the opposite in the Southern hemisphere.

The magnetic compass is difficult to,use in turbulence.

The compass should be swung frequently in order to keep the compass correction card up to date. If you notice serious compass errors report it on your Form 1-A. Keep metal objects and electrical equipment (such as headphones) away from the compass.

GYRO FLUX GATE

The Gyro Flux Gate compass system is a remote indicating earth-inductor compass consisting of gyrostabilized flux gate transmitter, an amplifier, a master indicator and from I to 6 remote indicating repeaters. It is not subject to the error of the magnetic compass except when the gyro flux gate is caged. A true heading is always indicated by the master and the repeater indicators, because of compensation for variation and deviation.

Its upset limits are 65° climb, glide and bank.

Although the responsibility for the operation of this compass rests on the navigator, you should know how to use it, and it is your responsibility if there is no navigator in the airplane. Follow these steps in operation:

1. Be sure the amplifier switch is always left "ON."

2. Turn on the inverter after starting engines.

3. Cage the gyro after the compass has been in operation 5 minutes.

4. Leave it caged for 45 seconds.

5. Uncage, and it is ready for use.

If you exceed the upset limits of 65°', cage and uncage the instrument just as you did before. Leave the gyro uncaged at all times except those indicated above. On the newer models the gyro is designed so it cannot be left in the caged position. Follow this procedure for erecting the gyro: After gyro motor has been operating 5 minutes, hold push button in until red light is on, then release push button. When red light goes out an erection cycle has been completed.

THE SUCTION SYSTEM

Gyro instruments are operated normally by suction, supplied by a vacuum pump driven by the airplane engine. Actually each gyro instrument works on the same principle as a turbo. A gage is provided to indicate amount of suction.

On modern single-engine airplanes there is no alternate source of suction. On multi-engine airplanes there are usually two or more suction pumps. A selector valve in the cockpit allows you to switch from one suction pump to another in case of engine or suction pump failure.

If your suction system fails completely you have approximately three minutes dependable use of the gyro instruments.

BANK-AND-TURN INDICATOR

The bank-and-turn indicator is a combination of two instruments; the bank indicator and the turn indicator. Since you use them together, however, they are housed in one case. The bank indicator consists of a ball free to move back and forth in a tube filled with a special liquid. It shows the relation between angle of bank and rate of turn. When a turn is properly executed the ball will remain in the center of the tube between the two reference lines. If the ball is on the low side you are making a slipping turn, if on the high side a skidding turn. You may use the ball in level flight to determine whether you are skidding or slipping, or if the airplane is properly trimmed.

The standard bank-and-turn indicator shows rate of turn and does not show the amount of turn. A needle width's deflection means that with a properly functioning instrument you are turning at the rate of 180° per minute. This is a standard rate turn used in instrument procedure.

The standard bank-and-turn indicator indicates rates of turn up to 1080° per minute.

The rate of turn is the result of two things-airspeed and angle of bank. The greater the airspeed the greater must be the angle of bank to produce a given rate of turn.

For a one needle-width standard turn (180° in one minute) the following speeds and angles are required:

It is obviously impossible to make angle-of-bank readings with sufficient accuracy in instrument procedure turns. But any of the above can be read on the bank-and-turn indicator as a single needle-width turn. At speeds above 200 mph make only one-half needle widths turns in order to avoid a dangerous angle of bank, providing the instrument has the standard calibration. To read the rate of turn in turbulence average the oscillations of the needle.

DIRECTIONAL GYRO

Use the directional gyro in connection with the magnetic compass. It is not a direction seeking instrument, and because of precession, must be checked every 15 to 20 minutes against the magnetic compass, and reset.

Set the desired heading by the caging knob underneath the dial.

Its approximate upset limits, are 55° bank, climb, and glide. When these limits are exceeded the card generally begins to spin; level the airplane, cage the gyro and reset by reference to the magnetic compass.

The instrument is reliable in turbulence, in contrast to the magnetic compass.

In ground checking the instrument turn the caging knob gently and pull out in one smooth operation. The card should stop and hold its position. This is a check on the rigidity of the gyro. Do not twist the knob sharply and pull it out at the same time.

If the instrument drifts off cardinal headings more than 3° in 15 minutes report it on your Form 1-A.

Leave the instrument uncaged at all times except in maneuvers which exceed its upset limits.

GYRO-HORIZON INDICATOR

This instrument indicates realistically the attitude of the airplane. The upset limits of the instrument are 100° bank and 70° climb and glide.

After starting engines allow at least 5 minutes at sufficient rpm before takeoff  for the instrument to obtain proper speed. Be sure before takeoff the horizon bar has settled into its proper 3-point position. This may be speeded up by caging and uncaging the instrument after engines are started. This is the only time the instrument should be caged while on the ground. Leave it uncaged at all times except in maneuvers which exceed its upset limits.

When caging and uncaging the instrument in flight always make sure the airplane is level both longitudinally and laterally at the moment of uncaging. Always cage the instrument gently to avoid damaging it.

When using the gyro-horizon be sure it is locked in the full "ON" position (uncaged).

Whenever you make a turn the gyro-horizon develops a small error in indication, known as turn error. After straight and level flight is resumed the error will disappear shortly. Therefore during turns and immediately thereafter refer to the altimeter and airspeed indicator to maintain altitude and compensate for the error in the gyro-horizon.

During flight do not depend solely on the gyrohorizon. Cross-check it with the other instruments.

 Reference: PIF 3-8-1ff