4.primary-and-supporting-instruments. Primary and Supporting Instruments
The primary and supporting method of attitude instrument flying is one of two recognized techniques (the other being the control and performance method) presented in the Instrument Flying Handbook for maintaining precise control of an airplane solely by reference to instruments. Under this method, every flight maneuver is broken down into pitch, bank, and power components, and for each component the pilot identifies the primary instrument — the one that gives the most direct, accurate, and immediate indication of the desired performance — and one or more supporting instruments that corroborate the primary indication and help anticipate trend.
The key concept is this: the primary instrument is the instrument that should remain constant (or change at a controlled rate) when the airplane is performing as desired. It is not necessarily the most sensitive instrument; rather, it is the instrument that directly displays the parameter being controlled. Supporting instruments indicate the attitude or power setting required to achieve and hold what the primary instruments are showing, and they alert the pilot to deviations before they appear on the primary.
Straight-and-Level Flight
In level, unaccelerated flight at a constant airspeed:
- Pitch — Primary: Altimeter. Altitude is what the pilot wants to hold constant; the altimeter directly shows whether that is being achieved. Supporting instruments for pitch are the attitude indicator (shows pitch attitude immediately, before altitude moves) and the vertical speed indicator (VSI) (shows pitch trend).
- Bank — Primary: Heading Indicator. A constant heading means wings are level. Supporting instruments are the attitude indicator (bank attitude) and the turn coordinator (rate of turn and slip/skid via the inclinometer).
- Power — Primary: Airspeed Indicator. At a fixed altitude and pitch, airspeed is the direct indicator of correct power. The supporting instrument is the tachometer or manifold pressure gauge, which shows that the throttle setting is approximately right.
Straight, Constant-Airspeed Climbs and Descents
When entering a climb or descent and after stabilization, primaries shift:
- During the entry, the attitude indicator is primary for pitch because it shows the pitch change being commanded.
- Once airspeed stabilizes at the target climb or descent speed, the airspeed indicator becomes primary for pitch (a constant airspeed in a climb means pitch attitude is correct for the power set).
- Heading indicator remains primary for bank.
- Tachometer or manifold pressure gauge is primary for power, because the pilot has set climb or descent power and is holding it.
Level Turns at a Constant Altitude and Airspeed
- Pitch — Altimeter (altitude must remain constant).
- Bank — Turn coordinator, when establishing and holding a specific rate of turn (typically standard rate, 3°/sec). The attitude indicator supports by showing the bank angle (approximately 15–18° for standard rate at typical light-airplane speeds, using the rule of thumb: bank ≈ TAS/10 + 7).
- Power — Airspeed indicator, since altitude and bank are being controlled.
Constant-Airspeed Climbing and Descending Turns
- Pitch — Airspeed indicator (after stabilization).
- Bank — Turn coordinator.
- Power — Tachometer or manifold pressure gauge.
How to Use the Method
The technique is built on a continuous cross-check (scan), interpretation, and control sequence:
- Cross-check — systematically sample the relevant instruments. Spend the most time on the primary, but never fixate; supporting instruments are scanned to detect trend.
- Interpret — translate raw indications into knowledge of the airplane's attitude and performance.
- Control — make smooth, small corrections with pitch, bank, and power. Corrections should be one-half the observed deviation, then trim.
Example: Holding 5,000 ft, 090°, 110 KIAS
The pilot's eyes return repeatedly to the altimeter (primary pitch). If the altimeter starts to creep up, the VSI (supporting) confirms a climb trend, and the attitude indicator (supporting) shows a slight nose-high attitude. The correction is a small forward pressure to lower the miniature airplane on the attitude indicator by perhaps half a bar width, verified by the VSI returning toward zero and the altimeter stopping. Heading is held by reference to the heading indicator (primary bank); a drift right is corrected by a small left bank shown on the attitude indicator, then rolled out as the heading indicator returns to 090°. Airspeed (primary power) is restored, if needed, with a small throttle adjustment confirmed by the tachometer.
Common Errors
- Fixation on a single instrument — usually the attitude indicator — to the exclusion of the primaries.
- Omission of an instrument from the scan, frequently the inclinometer or heading indicator.
- Emphasis — placing too much weight on a supporting instrument and chasing its indications instead of the primary.
- Making corrections that are too large, leading to oscillation around the target value.
Mastery of the primary and supporting method produces precise altitude, heading, and airspeed control and is the foundation for procedural instrument flying — holding, intercepting and tracking courses, and flying approaches.