Position Instruments

T-6A INSTRUMENT NAVIGATION

CHAPTER TWO

centrifugal force causes the ball to move to the inside of the turn. Correcting to coordinated

flight requires either decreasing the AOB or increasing the rate of turn by using more rudder or a

combination of both.

Angle of Attack (AOA) is the angle between the mean aerodynamic chord of the wing of a

moving aircraft and the relative wind. The AOA instrument is a visual indication of aircraft

performance. If the AOA is used to set the aircraft up for a phase of flight (e.g., maximum range

cruise, best rate of climb, optimum landing speed), many airspeed calculations can be saved.

Optimum AOA for any phase of flight does not vary with gross weight, bank angle, or density

altitude. AOA is measured by a sensor mounted on the outside of the aircraft. The sensor aligns

itself with the relative wind and transmits an electrical signal to the cockpit instrument, which is

a pointer needle mounted against a fixed dial. The instrument displays the AOA in numerical

units, degrees, or symbols.

A digital clock is installed in the instrument presentation and hours and minutes can be read.

The aircraft are equipped with clocks that have an elapsed time-counter feature.

Outside Air Temperature (OAT) (free air temperature) is indicated in the cockpit of many

aircraft. The temperature of the air mass surrounding the aircraft is shown in either degrees

Centigrade (C) or Fahrenheit (F). This information is useful for, but not limited to,

determining True Airspeed (TAS), true altitude, power required, and power available.

203. POSITION INSTRUMENTS

An altimeter is a flight instrument used to measure the height of the aircraft above a given

reference and display it on a calibrated dial. The reference may either be barometric (pressure

altimeter) or absolute (radio/radar altimeter).

During instrument flight below the transition level (18,000 feet in CONUS), the importance of

obtaining the latest altimeter setting cannot be overemphasized, particularly when flying from an

area of high pressure into a low-pressure area. Above the transition altitude (18,000 feet in

CONUS), pressure altitude is used since it is not as important to maintain true altitude as it is

indicated altitude. A mid-air collision is a more serious problem than terrain clearance.

Therefore, for high-altitude flights, the altimeter shall be set at 29.92" Hg climbing through

18,000 feet Mean Sea Level (MSL). The current local altimeter setting shall be set prior to

descent through the lowest useable flight level as defined in the current FLIP, GP.

The barometric scale located on the face of the altimeter is calibrated in inches of mercury and is

used to set a reference plane into the instrument. Setting the barometric scale to the altimeter

setting causes the altimeter to read indicated altitude. If the altimeter setting is given in

millibars, an appropriate conversion table must be used. Each .0l change on the barometric scale

is equivalent to 10 feet of indicated altitude. The altimeter must be checked and set prior to each

flight. This is accomplished as follows:

Dial the current altimeter setting into the barometric scale.

INTRODUCTION TO AIRCRAFT FLIGHT INSTRUMENTS 2-3