Under the Hood: Part 2: Basic Instrument Flight

In this second part of our series on instrument flying (with apologies for the long delay from the first article!), we’ll be looking at how to fly some basic manoeuvres using both the full and the limited panel.

Straight and Level

To fly straight and level we need five elements:

• Constant altitude
• Constant airspeed
• Constant direction
• Wings level
• Aircraft in balance

In visual conditions we identify each of these elements using outside references -- the position of the outside horizon in relation to the nose cowling, the distance of the wingtips from the horizon line and so on -- whilst the instruments can be consulted to confirm accurate flight.

With the outside view obscured, however, we must replace those external references with information gathered from the instruments.

Once again our control instruments are the attitude indicator and the power gauges. Just as in visual flight, we set the power required with reference to the engine gauges -- RPM, manifold pressure or EPR/N1. The flight controls are used to ensure the wings are level and the appropriate pitch attitude is set -- only this time with reference to the attitude indicator rather than the outside horizon.

The performance instruments allow us to monitor whether we are maintaining straight and level accurately and can be divided in to two groups - those giving information about the vertical performance of the aeroplane (the altimeter, VSI and ASI) and those giving information about the horizontal performance of the aeroplane (the HSI, turn co-ordinator and balance ball).

The altimeter directly indicates height. The VSI tells us about any trend away from that height. The ASI, meanwhile, indicates airspeed but indirectly provides information about height and pitch attitude - for instance, an increasing airspeed with a constant power setting implies a lower pitch attitude and, potentially, a loss of height.

As for the horizontal performance, the HSI, clearly, provides a direct indication of magnetic heading. However, indirectly it also tells us about our bank angle (because if the heading is changing the wings are almost certainly banked) and rate of turn (by comparing the change in heading with the time taken).

The turn co-ordinator directly indicates rate of turn, and in combination with the balance ball provides indirect clues about bank angle (because if there is no rate of turn and the balance ball is centred, it follows that the wings must be level).

To maintain straight and level, we use exactly the same sequence as for visual flight -- P-A-T.

• Power is set to the cruising setting with reference to the power indicators
• Attitude is set to the level flight attitude using the elevator with reference to the attitude indicator
• Trim the aircraft to relieve any control forces required to maintain the selected attitude

Straight and level flight is controlled by setting the correct pitch attitude on the ADI and the correct cruise power on the power gauges. Vertical performance is monitored using a selective radial scan of the altimeter, VSI and, to a lesser extent, the ASI.

The altimeter is used to ensure that the correct height is being maintained. It will also indicate deviations from this altitude (and information about the rate of change of height can be gathered from the speed of movement of the altimeter needle).

The VSI is very sensitive and will often register a rate of change of height before any significant movement of the altimeter. However, remember that both the VSI and the altimeter are subject to instrument lag and so great care must be taken to avoid overcorrecting and ‘chasing’ the indications back and forth.

Small corrections for altitude deviations of less than 100 ft can be accomplished using pitch alone. The change in pitch attitude required is generally very small, in the order of only a single degree or so on the pitch ladder.

• Change the attitude slightly in the direction the correction is required
• Check the control pressure to hold the new attitude steady on the ADI
• Hold the new attitude and verify the correction is having the desired effect
• Trim to relieve the control pressure

Deviations greater than 100ft will require a more positive correction and potentially an increase or decrease of power as well.

However, even if the altitude is changing rapidly, the correction should still be smooth and progressive, with light pressure on the controls. The total pitch correction required, however, may still only be in the order of just one or two degrees.

Instrument flying requires a light touch on the controls; this requires you to be relaxed and the aeroplane to be well-trimmed. Should you feel tense on the controls, one useful technique (provided the aeroplane is in trim) is to release all pressure on the elevator control for a moment or two and flex your fingers before replacing them on the controls. The thumb and first two fingers are more than sufficient for adequate control: a full hand grip may lead to lack of sensitive control and further tenseness.

Limited Panel

Whilst modern instruments are generally very reliable, there can be many reasons for them to fail or indicate incorrectly, a situation we are unfortunately well aware of from a number of aircraft accidents over recent years.

Vacuum Failure

One of the most common reasons for instrument failure in light aircraft is a failure of the vacuum system. This is because the gyroscopes which operate the artificial horizon and direction indicator are typically air-driven whilst the turn co-ordinator gyro is normally electrically-driven to provide redundancy. As such, most limited panel flying considers the failure of the artificial horizon and the gyro direction indicator, though other failures (for example, of the pitot-static system which would affect airspeed, altitude and vertical speed indications) are also possible (though, provided the artificial horizon is operating, relatively straightforward to deal with if the failure is correctly identified).

Identification

If an instrument failure is suspected, the inverted V scan can be used to determine the source of this issue. Here the artificial horizon, VSI and turn co-ordinator are scanned and their indications compared. These three instruments are typically driven by separate systems (the AH by a vacuum-driven gyro, the turn co-ordinator by an electric gyro and the VSI by the static system) so a failure of any one of the systems will result in that instrument displaying conflicting information.

Inverted V scan

One of the major challenges of dealing with instrument failure is disregarding the false indications of the failed instrument, which can be very distracting and enticing! For this reason it is not a bad idea to physically cover the failed instrument if possible, for example using a post-it note or similar.

Straight and Level

Without the artificial horizon, pitch and bank attitude information must be inferred through the use of the performance instruments. As we already know, provided the power is set correctly the altimeter, VSI and ASI provide a good guide to the aircraft’s pitch attitude: if the airspeed is increasing and the height on the altimeter reducing, the pitch is too low, and if the airspeed is increasing and the height on the altimeter increasing the pitch is too high.

The ASI and altimeter can be used to infer information about pitch attitude

Bank attitude can also be inferred using the turn co-ordinator and magnetic (‘wet’) compass. If the wings on the turn co-ordinator are level and the ball is centred, the aircraft’s heading is not changing and the wings must be level. This can be confirmed on the wet compass, which should also be unchanged.

The turn co-ordinator be be used to infer information about bank angle. This is an example of an 'unusual attitude' - can you work out what corrections need to be made? Answer at the bottom of the article...

Turns

To make a turn on the limited panel, gently roll the wings using the ailerons in the direction of the desired turn, remembering to keep the ball centred with the rudder. As with all turns in instrument flight, the aim is to turn at Rate 1, so once the wings of the turn co-ordinator line up with the Rate 1 markings, neutralise the ailerons, use rudder to keep the ball centred and make small adjustments with the ailerons to maintain Rate 1, all the while keeping your scan of the altimeter, VSI and ASI going to maintain height.

One of the advantages of flying a Rate 1 turn with a failure of the DI is that you can time the turn -- at three degrees per second a 180 degree turn will take one minute, a 90 degree turn 30 seconds and so on (allowing for time to roll in and out). So if you start a stopwatch as you roll in to the turn you will have a good idea of how long you will need to maintain the turn for.

The wet compass is also used; remember however that the compass is subject to errors when turning through north or south. In the northern hemisphere, these can be easily remembered using the acronym UNOS: Undershoot North, Overshoot South (the situation is reversed in the southern hemisphere to give ONUS). These errors are quite well-simulated in MSFS/P3D.

What this means is that if you are turning, say, from west to north, the wet compass will lag behind the actual heading so you will need to start rolling out well before the compass indicates the target heading.

How far before is dependent on your approximate latitude: for example, at a latitude of 55° north you would need to start rolling out when the compass indicates 55 degrees before north, plus half of the bank angle (to allow for the roll-out).

If the bank angle is not directly known (because the artificial horizon is unavailable) then the bank angle for a standard rate turn can be estimated by dropping the last digit of the airspeed then adding five -- for example, at 120 knots drop the zero = 12 and add five = 17 degrees.

In this example, the roll-out would need to be commenced when the compass indicates indicates 296° -- 360 - 55 = 305 (for the latitude) minus a further nine degrees to allow for the roll-out (approximately half the bank angle).

The reverse is true when turning on to a southerly heading -- here the wet compass will overshoot and so the roll-out will need to be commenced significantly after the desired heading is indicated (for the same turn from west to south, the roll-out would need to be started when the compass indicates 134°). The error reduces linearly towards east and west headings, so for example turning from 270° (W) to a heading of 225° (SW) the compass lead due to latitude would only be half the value when when turning to 180° (S). Turning on to east or west, there is no compass lead/lag.

The secret to flying on the limited panel, as with all instrument flying, is to fly with a light touch and make only small corrections. Remember that the pressure instruments suffer from a degree of lag and the effects of inertia -- a change in pitch attitude will take some time to affect the airspeed and so on. So remember to be smooth and gentle in your inputs, and wait for any change to take effect and for the instruments to settle down before making a further change -- it is very easy to get in a pickle if you start making large and alternating inputs as the instruments, unable to keep up with the changes you are making, will end up displaying very confusing and rapidly-changing indications!

Summary

• Fly with a light touch on the controls - just your thumb and the first two fingers are sufficient!
• Be smooth and gentle in your inputs; change - check - hold - trim
• Keep the aircraft well-trimmed at all times
• Consider physically covering any failed instruments to avoid distraction
• For compass turns - UNOS (Undershoot North, Overshoot South) (ONUS in southern hemisphere)

In the next article (which I promise will follow much sooner than this did the first!) we’ll take a look at radio navigation and tracking using the ADF.

Oh - and the answer to the question about the attitude shown in the limited panel example above? Here's the ADI...

Hopefully you don't end up like that when you're practicing!