Instrument Flying E-Book

David Hoy

• Herausgeber: Crowood
• Kategorie: Fachliteratur
• Sprache: Englisch

Seitenzahl: 307

Veröffentlichungsjahr: 2014

Beschreibung

This book is designed to supplement the instruction a student receives during his or her course. The Instrument Rating is a test of not only the student's ability to fly accurately on Instruments, the foundation, but also the ability to cope under a number of pressures. Instrument Flying is intended to help prepare the student to pass what is regarded as probably the most demanding flight tests in the world, the JAA Instrument Rating. It will also provide some useful tips and reminders when the IR renewal is due. Subjects covered include: Instrument Flight; Use of Radio Navigation Aids; Let Down and Approach Procedures for both ILS and NDB; Airways Flight; The Instrument Rating Test. Illustrated with between 20 - 30 exercise charts.

Leseprobe

Instrument

Flying

David Hoy

First published in 1995 by Airlife Publishing, an imprint of The Crowood Press Ltd Ramsbury, Marlborough Wiltshire SN8 2HR

www.crowood.com

This e-book first published in 2014

Revised edition 2007

© David Hoy 1995 and 2007

All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopy, recording, or any information storage and retrieval system, without permission in writing from the publishers.

British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library.

ISBN 9781847979247

CONTENTS

Introduction – The JAA Instrument Rating Skill Test (A)

Chapter 1

The Instrument Rating

Chapter 2

Instrument Flying

Chapter 3

The Day Comes…

Chapter 4

The Departure

Chapter 5

En-route IFR Procedures

Chapter 6

The Precision Approach

Chapter 7

The Asymmetric

Chapter 8

Tracking and Holding

Chapter 9

The Non-Precision Approach

Chapter 10

General Handling

Chapter 11

The Flight Test Flown!

Appendix I

Definitions

Appendix II

Abbreviations

Index

DEDICATION

This book is dedicated to my wife, Elaine, my two children, Anna and Catherine, and my good friend Richard Darlow.

INTRODUCTION – THE JAA INSTRUMENT RATING SKILL TEST (A)

This book is designed to supplement, not replace, the instruction you will receive during the simulator and flying training of your course. It is not intended that it should explain in detail the technical aspects of the various navigational systems or flight instruments, although these will be touched upon. It reflects the experience gained during a lifetime of specializing in Instrument Rating training.

The course you are about to start, or may have started already, to obtain the Instrument Rating is exacting, but enjoyable. It’s a test not only of your ability to fly accurately on Instruments, the foundation, but also your ability to cope under a number of pressures, one of which may be financial. Of course, no two flights are ever the same. No matter how well prepared you are, surprises will occur and you should expect them, as much as anyone can expect the unexpected! The key thing to remember is that this is a test akin to the task of a one-armed paper hanger, and good preparation is definitely the key. Not much help will come from the examiner’s seat during the flight, nor should any be sought. To keep the metaphors running, you are paddling a one-man canoe!

This book is intended to help prepare you for what is probably the most demanding flight test in the world, the JAA Instrument Rating. Moreover, I hope it will help you achieve a much coveted first-time pass! It will also provide some useful tips and reminders when your IR renewal is due, in a year’s time.

Subjects covered include:

•

Instrument flight

•

Use of radio navigation aids

•

Let-down and approach procedures for both ILS and NDB

•

Airways flight

•

The Instrument Rating Test

I hope that you will find this book both enjoyable and useful, and most importantly, I trust it will save you money.

My thanks are due to all those who have supported and helped me in the writing of it, particularly the Oxford Examiners – Ian McClelland, Nigel Ashley and Steve Oddy – and their boss, Chief Flight Examiner Pat Lander. Thanks also to Aerad for allowing me to reproduce their approach plates and charts. The views expressed within this publication are the author’s and not necessarily those of the Flight examiners mentioned above.

Finally, thanks to my wife Elaine and to my friend Richard Darlow, both of whom encouraged me to write this third edition. Incidentally, no animal was harmed or mistreated in the writing of this book, except the cat, which received the occasional kick when I hit writer’s cramp!

NB: No Aerad chart or approach plate published in this book should be used for navigational purposes. If any statement in this publication is at variance with your school, instructor or pilot’s operating handbook, it should be ignored.

CHAPTER 1

THE INSTRUMENT RATING

TRAINING REQUIREMENTS

Without an Instrument Rating (IR) in your licence, you will not be employable as a commercial pilot, unless you are considering flight instruction, glider towing, crop spraying or perhaps aerial photography as a career! An IR permits entry into controlled airspace under Instrument Flight Rules, an essential for the airline pilot. Training for the IR must be in accordance with JAR FCL 1 and must be approved by the Civil Aviation Authority. The test, however, must be taken within a JAA JAR FCL approved state and, in the UK, with a CAA employed flight examiner.

The multi-engine IR (A) course (for aeroplanes) has the following minimum requirements:

1.

55 hours instrument time under instruction, which may include 2 or 3 below, but must include 4.

2.

25 hours (maximum) in an FNPT 1 if approved by the CAA.

3.

40 hours (maximum) in an FNPT 2 or flight simulator if approved by the CAA. With the agreement of the authority, no more than 10 hours of this 40 can be done in an FNPT 1.

4.

15 hours in a multi-engine aircraft.

Holders of a CPL (A), or those who have passed a CPL skill test and have met all the requirements for a CPL issue, can have the 55-hour minimum reduced to 50 hours.

The flight test for the Commercial Instrument Rating is usually carried out in a multi-engined aircraft, but not a centre-line-thrust, two-engine machine. A candidate for the flight test should either hold a multi-engine class rating or have passed the test for such a rating within the preceding six months.

More comprehensive information on training requirements and flying experience requirements can be obtained from the latest copy of Standards Document 1 Appendix D. Don’t leave home without it!

TEST PROFILE

The idea that the IR test’s standard profile comprises en-route, ILS, EFATO, asymmetric hold, NDB and general handling is incorrect. Some test centres frequently fly asymmetric ILS approaches because such profiles allow them to get the job done more efficiently. At Oxford, for example, there are few situations where it becomes imperative to fly an asymmetric ILS, although such situations may arise because of the reluctance of some airfields to allow procedural approaches. Consequently, applicants should not be surprised to have to fly an asymmetric ILS if the situation demands it.

Similarly, radar vectored NDB approaches may be flown, although it is more common for these to be flown from a procedure. Because of the problems of removing screens at DA/DH and the proximity of ACA, the asymmetric ILS is likely to lead to an asymmetric IFR go-around. If a visual circuit is then to be flown, the examiner will remove the screens once the aircraft is climbing safely and re-orient the applicant as necessary before the turn downwind.

The Test

The IR test is divided into six main sections:

1.

Pre-flight operations and the departure.

2.

General handling on instruments.

3.

Fn-route IFR procedures.

4.

Precision approaches, go-around or landing.

5.

Non-precision approach, go-around or landing.

6.

Asymmetric flight.

Either Section 4 or 5 must be flown following an ATC procedural clearance. The other approach may be carried out by following radar vectors. A hold will be a requisite on either Section 4 or 5.

The Instrument Rating initial flight test, as taken in the UK, must be conducted by a CAA employed flight examiner. It is designed to simulate as closely as possible a real trip.

Typically, the test will be based on a public transport flight from aerodrome A to airport B. At B, a radar vectored ILS will be flown, but on reaching decision altitude, the runway will be deemed not visible, and a go-around and missed approach will be required. During the go-around, an engine failure will be simulated. The necessary drills must be followed and the decision made to divert to airport C, where a hold and a non-precision asymmetric approach will be performed, eventually to an asymmetric committal altitude. This will be followed by a visual asymmetric circuit to a landing. At some stage during the flight, the examiner will want to see some limited-panel flying, including turns on to specific headings and recoveries from unusual attitudes, as well as two stall recoveries in full panel.

I should point out that not all flights will follow this pattern. Due to availability of radio aids, you may find yourself flying an asymmetric ILS approach and a symmetric NDB or non-precision approach. The examiners are not restricted to a set format. The good news is that all the aerodromes A, B and C will be known to you before you depart, and the format of the test will have been briefed in detail. Only the unforeseen or unplanned will affect the schedule. Fingers crossed, it should go to plan!

The flight examiner will choose a route for the test that may, or may not, start and end at the same aerodrome, but will not normally exceed a distance of 150 nautical miles. Typically, the flight should not last more than two-and-a-half hours.

PASS OR FAIL?

All sections of the test must be completed within six months. For the purpose of the IR issue, the skill test remains valid for six months. Prior to taking the actual test, a candidate is required to obtain what is known as the 170A in the same class of aircraft. This is achieved by flying with an authorized 170A flight instructor, who will confirm on the form his confidence that you stand a good chance of passing the test, that he has verified your hours and that you are entitled to fly with the CAA. You should also have passed all the appropriate examinations. Exceptions can be made if you’re on an integrated course.

An IR test allows two attempts at completion, assuming a partial pass was achieved on the first attempt. A partial pass is granted when only one section is failed at the first attempt. Failure of more than one section is considered a full fail, requiring a complete retest at the next attempt. All ‘partials’ require a re-examination of Section One in addition to the section actually failed. If on the ‘partial’ retest, you fail to make the grade again, a complete new test must be taken, with all sections being examined. If you achieve a complete fail on your first attempt, even though you may have passed some sections, you will be examined on all sections at your next attempt.

Further training may be required following any failed test. Failure to achieve a pass in all sections of the test in two attempts will require further training as determined by the Authority. There is no limit (other than financial) to the number of attempts you are allowed at the Instrument Rating.

From this, you should appreciate that the most important section is the last one. Suppose, for instance, that you have inadvertently failed the departure, but the rest of the flight has been perfect as you approach the final section. Everything hangs on that final section. You must give it your very best to ensure that you walk away with a ‘partial’ and not a complete fail.

School report form.

CAA F170A certificate of training and competence.

CAA Instrument Rating skills test report form.

CAA application form for attachment of Instrument Rating to existing licence.

CAA application form for revalidation or renewal of an Instrument Rating.

ESSENTIAL CONSIDERATIONS

Tests for the Instrument Rating normally occupy a full morning or afternoon. Traditionally, there are two reporting times: 0845hr and 1245hr.

Prior to the reporting time, your flying instructor should ensure that all the necessary paperwork is in order. You should be in possession of form FCL 170A. This states that, in the opinion of the person who has signed it, you are ready and able to pass the flight test. Normally, the 170A flight is one of the last you will make prior to sitting the actual Instrument Rating test. Ideally, you will have been introduced to the examiner by your instructor on the previous day, and will know where his office is and where you will be doing your planning. This planning should be unassisted!

You should hold a current medical certificate (a licence is not necessary) and be able to show proof that either you hold a multi-engine class rating or have passed the test for such a rating in the previous six months. If your medical has lapsed, you may still be allowed to take the test at the discretion of the examiner.

The aircraft you intend using for the flight test should have been approved by the Authority. This involves having a CAA examiner inspect the aircraft, its paperwork and its blind-flying screens so that an approval form can be issued. Such forms are valid for twelve months and can only be renewed by a flight examiner or someone designated by the CAA.

At this stage, the examiner will discuss briefly the weather with you, and give you the route he wishes to fly and various key times that will be needed for your flight plan.

Planning

About an hour is allowed for planning. During this time, you will be expected to file your flight plan, obtain all the necessary weather (departure, destination and en-route), and compute headings, timings etc. for the route and the various procedures that you will be expected to carry out during the flight. A weight-and-balance calculation, a fuel plan, and performance calculations for departure and arrival should also be completed, and a copy given to the examiner at your next meeting, when the main briefing will be given.

Flying Accuracy

Rules are for the guidance of wise men etc. The limits that follow are subject to the examiner’s discretion. One error will not necessarily mean that you have failed the test, especially if the limits have been observed throughout the rest of the flight.

Height control

+/– 100ft.

Heading

+/– 5°, both engines operating.

+/– 10°, one engine operating.

Radio aid tracking

+/– 5°.

Speed

+/– 5kt, both engines operating.

+ 10/– 5kt, simulated engine failure.

ILS

+/– half-scale deflection.

If you make an error, be seen to correct it. If you are 75ft high, don’t just sit there; strive for perfection, all the time. Remember high and fast, slow and low are always power problems!

Performance

Examiners require both performance and mass-and-balance calculations needed for the planned public transport flight. That means mass and balance at take-off with a projected line showing mass and balance for the rest of the flight; take-off performance related to departure aerodrome; landing performance related to your planned destination, or landing performance for the intended diversion if it is clear that this would be the most limiting case (for example, destination Birmingham, diversion Gloucestershire). While you should calculate TODR and LDR correctly, make sure you have checked these against TORA or LDA. Otherwise the calculations will be meaningless. Make sure you obtain the correct LDA for your calculations. All the examiners require is a real-world understanding of these calculations and their implications.

Aerad airfield plate gives essential information when calculating take-off and landing performance.

Typical weight and centre of gravity schedule, and performance graphs. These are for the PA34 Seneca.

CHAPTER 2

INSTRUMENT FLYING

Many good books have been written about instrument flying, and in the course of this particular book, much of what you’ve read before will almost certainly be repeated. I make no apologies. My hope is that this book will serve many purposes. It should help not only the low-houred student pilot, but also the more experienced aviator about to take or renew an Instrument Rating. It may even assist those who are already qualified and simply want a manual to refresh their knowledge. Its primary aim is to serve anyone who is on the road to the JAA Instrument Rating and who could benefit from a few good tips and ideas.

No pilot should ever be a good visual flyer but poor instrument flyer. Exactly the same principles apply to accurate instrument flying as to accurate ‘natural horizon’ or visual flying. This is an important principle to grasp. No different methods are employed for flying straight and level in the local training area, regardless of whether the pilot is looking out at a cloudless sky or has a set of screens obscuring the outside world. Basic technique does not change. An aircraft is an aircraft, whether it’s flying in total fog or in eight-eighths blue! The wings still produce the same lift, and power plus attitude always equals performance, just as lift will always equal ½ pV2SCl.

As long as that fact is appreciated, the next conclusion is obvious. To have the correct foundations for good instrument flying (IF), your basic general handling skills must be sound. If these techniques are faulty, they will be carried through to your instrument flying, so beware. Before you try to impress yourself and your instructor with your ability to stay the right way up, on the correct heading and at the right speed, without at least one eye on that ever-encircling horizon, make sure you can do it properly visually.

In the United Kingdom, there are two qualifications that indicate your instrument flying abilities: the IMC (Instrument Meteorological Conditions) Rating and the Instrument Rating. The former is peculiar to the UK, and although many will disagree, I feel that the amount of training required to obtain this qualification does not sensibly entitle the holder to search out the type of weather it entitles him to fly in. The Instrument Rating does. More on this later. No training is wasted, however, and the more training you do, the more proficient an instrument pilot you will become. Instrument flying is also an ability that quenches its thirst on currency. If a pilot has not flown on instruments for several weeks, he should not expect to be as accurate as the pilot who is in constant practice. Again beware!

THE INSTRUMENTS AND HOW THEY WORK

While modern airliner-type aircraft have had their instruments amalgamated in the glass cockpit philosophy, the basic scan technique and use of these instruments remain the same. Combined with these, a good ‘layman’s’ knowledge of how the instruments work, their occasional faults and problems, and their inaccuracies is useful to the proficient pilot. When flying in IMC, the accurate interpretation of the aircraft’s flight instruments is essential. To some extent, this skill depends upon a basic understanding of how they work.

THE GYROSCOPIC FLIGHT INSTRUMENTS

There are three gyroscopically operated instruments in most training aircraft:

•

The attitude indicator (AI, artificial horizon).

•

The directional gyro indicator (DGI).

•

The turn co-ordinator (replaced the old turn-and-bank indicator).

AI, DGI and turn co-ordinator.

These instruments are driven by a combination of air pressure or suction and electricity. Normally in small aircraft, the attitude indicator and the DGI are air driven, while the turn co-ordinator is powered electrically. This system offers a back-up if one source of power fails. ‘Belt and braces’ is a good aviation philosophy; if one important system fails, there should always be a back-up available.

Gyros have two main attributes: rigidity and precession. The former is demonstrated when the gyro is turning – its axis will remain in a fixed position unless an external force is applied to it. This property enables us to measure changes in both the attitude and the direction of the aircraft.

When a force is applied to a gyro’s axis, the resultant movement of the gyro is known as precession. This movement makes it appear as if the force had been applied at a point 90° displaced from the actual point of pressure and in the same direction that the gyro is turning. This principle is used in the ‘righting’ of the attitude indicator and also in the measurement of rate of turn on the turn co-ordinator. Sometimes, precession can be unwanted, as it can cause errors, which must be understood.

THE ATTITUDE INDICATOR (ARTIFICIAL HORIZON)

Rather like a submarine commander’s periscope, the attitude indicator gives the pilot an immediate indication of what’s happening to the aircraft relative to the natural horizon. This is the only instrument that shows both pitch and roll directly. Pitch and bank changes are conveyed by the displacement of a miniature aircraft symbol in relation to the instrument’s horizon line. In addition, a pointer, normally at the top of the instrument, displays the angle of bank. The attitude indicator is known as a control instrument.

Attitude indicator (‘artificial horizon’).

The attitude indicator is an earth-tied gyroscope, the axis of the gyro being aligned with the earth’s gravity. Initial erection is assisted by the instrument being pendulously weighted with four vanes that will cause the gyro to return to its correct setting if displaced during flight. When an attitude indicator is started, the pendulous device reacts to gravity and causes the gyro to erect vertically by means of the erection force, either an air blast from the pendulous vanes or an ‘electric torquer’. This alignment with gravity is called the ‘local vertical’. In unaccelerated level flight, the pendulous sensor reacts to the force of gravity and directs the erection mechanism to align the gyro with the local vertical, removing precession.

When the aircraft is in a co-ordinated turn, the sensor will falsely detect gravity in the same way that we humans do, and will try to erect the gyro to an incorrect angle. However, the erection force available is very small compared to the rigidity of the gyro, so there is little change to the gyro angle during the short period of a turn. When the wings are rolled level again, the pendulous sensor is accurate and goes to work removing any precession that has developed.

The system would not work if the aircraft were turned constantly or flew very shallow angles of bank over long periods of time. A gyro would be a useless attitude indicator, however, if it were not referenced to gravity, because, as pilots, we need to determine up from down, and gravity is down.

A gyro, remember, remains rigid in space. A gyro free from any precession placed on your desk would, twelve hours later, appear to be upside-down. In fact, you would be upside-down relative to the gyro and not the other way round. The only way we can use gyros, therefore, is by constant reference to the local vertical by means of a gravity sensor.

Errors

As already discussed, prolonged turns can result in errors, although very small. These errors are amplified, however, if a prolonged steep turn is flown. In this case, when the aircraft is returned to straight-and-level flight, the instrument will indicate a slight climb and a turn in the opposite direction. An unbalanced turn, such as a skidding turn, will cause the gyro to precess towards the direction of the turn, but again the error is small.

Acceleration and deceleration errors are rarely seen on the small, relatively low-powered singles and twins that are generally used for Instrument Rating training. However, acceleration can result in the attitude indicator showing a climbing turn to the right, and deceleration a descending turn to the left.

All attitude indicators need a constant power supply, be it air or electricity. Relying principally on the property of rigidity, they must sustain high rotational speeds for the gyro. Any loss of suction may jeopardize the safe use of this instrument. Include the suction gauge in your scan. Any failure of the suction system is normally insidious and gradual. Imagine slowly following a toppling attitude indicator!

Pilot Interpretation

Misinterpretation of the attitude indicator, although not common, can result in dangerous situations developing. Make sure you are fully conversant with all the information the display offers. This instrument takes time to achieve rigidity and reach a ‘working’ state. Aircraft have been known to become airborne too quickly following start-up, with inevitable and disastrous results.

THE DIRECTIONAL GYRO INDICATOR (DGI)

The DGI is normally a suction driven instrument, which senses movement about the vertical axis, the gyro having a horizontal axis of rotation. In some light aircraft, an electric sensor movement causes the gyro to remain ‘slaved’ to Magnetic North. Such a gyro is called a slaved gyro. Most aircraft used in IR training will be so equipped. Normally, what is known as a ‘magnetic flux detector’ is located in a wing or sometimes the tail of the aircraft to sense the earth’s lines of magnetism. The DGI uses the principles of rigidity to maintain its alignment; a single gimbal allows the gyro to sense changes in direction.

Errors

Like any other gyro, the DGI suffers from precession and also from the phenomenon of drift – as a result of the earth’s rotation and from what is known as ‘transport wander’. Both of these problems are beyond the scope of this book to explain in full. Suffice to say that an unslaved gyro needs to be checked every fifteen minutes or so, and aligned with Magnetic North. Incorrect air supply to the DGI will, of course, impair the instrument’s accuracy. Again, the gyro requires a high revolution rate to maintain its rigidity, typically 18–20,000rpm.

THE TURN CO-ORDINATOR AND SLIP INDICATOR

This instrument enables the pilot to monitor an accurate rate of turn and determine that the turn is in balance. A rate-one turn equates to a change of heading at the rate of 3° per second, 180° per minute. The required bank angle is determined by true airspeed (TAS). The faster you fly, the larger the bank angle required to achieve a rate-one turn. A useful rule of thumb is to take 10 per cent of your true airspeed and add seven; that will give you the bank angle required to achieve a rate-one turn.

For example, if TAS is 120kt, the bank angle for a rate-one turn will be 19° (12 + 7).

The turn co-ordinator displays both yaw and initial roll by utilizing a gyro with an axis of spin that is not quite horizontal, thereby being susceptible to both yaw and roll. The older turn-and-slip/bank indicator used a simple gyro with a horizontal axis of spin that detected yaw only, not roll. It should be understood that a turn co-ordinator is set for one particular forward speed. An aircraft flying at a different speed will result in the instrument’s indications being not entirely accurate, although the differences are normally too small to be of concern.

Errors

The main errors suffered by the turn co-ordinator are ‘looping error’ and ‘lag’. When subject to positive G during a yawing manoeuvre, the instrument will over-read. If subject to negative G, it will under-read. This is a good reason to level the wings prior to applying G in a recovery from an unusual attitude. Otherwise, the pilot could bring his wings level to a false horizon according to the incorrect turn co-ordinator. We compensate for lag by rolling through the wings-level position before centralizing the aileron control. More on this later.

The Balance Indicator

The position of the ball of the balance indicator shows the pilot whether the aircraft is in balance, that is neither slipping into a turn, nor skidding out of it. For a turn to be balanced, the aircraft’s local centre of gravity will be perpendicular to the plane of the wings. In a way, the ball shows you just how well a turn is being flown – is the correct amount of rudder being applied?

During a turn, the horizontal component of lift acts towards the centre of the turn, thus opposing centrifugal force. If the turn is co-ordinated, i.e. in balance, the horizontal component of lift is exactly balanced by the centrifugal force, and the ball remains centred. If the aircraft is skidding in the turn, the centrifugal force exceeds the horizontal component of lift, and the ball becomes displaced away from the direction of turn. This shows that the rate of turn is too large for the angle of bank. Therefore, more bank is required or less rudder.

Balance indicator.

In a slipping turn, the opposite applies. Too much bank is being applied for the rate of turn, and the ball moves towards the centre of the turn.

Load factor, the ratio of lift to weight, is affected by the co-ordination of a turn. A skid produces a greater centrifugal force, leading to an increase in load factor and also an increase in stalling speed. The opposite applies to a slipping turn. Pilots should always seek to keep the ball centred for comfortable, co-ordinated flight.

GYROSCOPIC INSTRUMENT CHECKS

Prior to any flight, an instrument check should be made. If the flight is intended to enter cloud or low visibility, it is essential for the pilot to know that the instruments are serviceable and likely to stay that way. Pilots should not forget that gyroscopes take a certain time to ‘wind up’ and reach their operating speeds. Normally, this takes a matter of minutes. Flight should not be attempted until the gyros have reached their correct working speed. A checklist followed correctly will normally ensure this.

During taxiing, turns should be made in both directions. During these manoeuvres, the turn co-ordinator should correctly show the direction of turn, while the ball should swing out in the opposite direction. The DGI should also show a turn in the same direction, as should the radio magnetic indicator (RMI), if fitted. If the turn co-ordinator has a ‘fail flag’, this should not be visible. Check also that the stand-by compass is turning freely and correctly, and has no trapped bubbles or fluid discoloration. During start-up, the attitude indicator should erect fairly quickly – within thirty seconds – and indicate wings level during turning, and a slight pitch down during braking.

After start-up, align the DGI, if unslaved, to the stand-by compass and recheck it prior to departure. Precession should not be more than 5° per fifteen minutes.

Do not forget the power sources for these instruments. Check for adequate suction and that ammeters indicate correct charging prior to take-off. Some aircraft have warning flags or ‘dolls’ eyes’ for suction failure; check that these are not visible. Ensure the turn co-ordinator is not showing a fail flag to indicate a failure of the electrical supply.

A typical instrument check after confirming correct brake operation and full-and-free rudder would be as follows:

Turning left: turn co-ordinator – turn left, skid right; horizontal situation indicator (HSI) decreasing; RMI decreasing; compass decreasing through east; ADF tracking; AI erect and stable.

Note that the check is made through east. Why? Well, there are no turning errors through east, and the stand-by compass should read as accurately as the DGI and the RMI.

THE MAGNETIC COMPASS

Sometimes known as the E2b-type or stand-by compass, the magnetic compass is a directional seeking instrument. While the device can give relatively accurate information, it is important to have an understanding of its faults and weaknesses.

Deviation

The difference between the correct magnetic heading and what the compass actually shows to the pilot is known as deviation. In any cockpit, it is difficult to completely eradicate spurious magnetic fields that will have an unwanted effect on the compass. It is the job of the engineer to ‘swing’ the compass and reduce the errors to a minimum, and to take notes on a compass card of any remaining errors. Deviation is never constant on all headings and can be affected by the introduction of radios or even headsets into the cockpit. Avoid leaving your headset too close to the compass. Check that the deviation card is in place for the compass and, if fitted, the HSI. Each should have its own deviation card.

Magnetic Dip

The vertical component of the earth’s magnetic field causes magnetic dip. There is almost no dip at the earth’s magnetic equator, but it is at the maximum at the magnetic poles. Dip gives rise to acceleration and deceleration errors, and also to turning errors.

Acceleration/deceleration errors are the observed fluctuations in heading when an aircraft either gains or loses speed. In the northern hemisphere, an apparent turn to the north is observed during acceleration, and an apparent turn to the south during deceleration. The opposite applies in the southern hemisphere. These errors are worse when changing speed in an east/west direction, and are zero in a north/south direction.

Turning Errors

When turning through south or through north, turning errors are at a maximum. There are no errors when turning on to east or west. These are good headings to check when taxiing. Turning errors increase towards the magnetic poles, and are at a minimum at the magnetic equator. When turning on to a northerly heading using the compass, you should undershoot the required heading; when turning on to a southerly heading, you should overshoot.

The question is by how much should you under- or overshoot? This is dependent on latitude. Typically in the United Kingdom, an anticipation of 30° will work as the lead or lag figure. For example, if turning on to heading north from west, level the wings on a heading of 330°; if turning on to south from east, level the wings on a heading of 210°.

In the southern hemisphere, turning errors occur in the opposite sense.

THE PRESSURE INSTRUMENTS

The pressure instruments include the altimeter, the airspeed indicator (ASI) and the vertical speed indicator (VSI). Static or ambient pressure is used by all the pressure instruments, but pitot or dynamic pressure is used only by the airspeed indicator. The pressure instruments are known as the performance instruments.

Checking the Pressure Head

Prior to taxiing, ensure that all holes in the pitot or pressure head are clear. In addition, confirm that the static vents, if fitted, are clear. Check also that the pitot heat is serviceable, but don’t leave it switched on for too long, as it is a tremendous drain on the battery, and the elements will burn out quickly if there is no cooling airflow over the pitot. A typical checklist will delay switching on the pitot heat until just before take-off. The pitot heat switch may also control heating of the stall warner vanes, as in the Seneca 11.

THE AIRSPEED INDICATOR

The ASI measures the aircraft’s speed through the air by making a comparison between dynamic pressure and static pressure. Dynamic pressure is made up of pitot pressure and static pressure. Pitot pressure is the pressure that results from bringing a moving mass of air to a halt within a capsule. However, the pressure that is received in the capsule within the ASI includes static pressure as well. Since static pressure varies with air density or altitude, it is important to remove this static pressure from the calculations. This is achieved by allowing static pressure to enter the ASI surrounding the capsule, which then negates the unwanted effect of varying static pressure with height.

There are a number of different ‘speeds’ that pilots need to know and understand:

Vso is the stalling speed in the landing configuration, at the maximum landing weight with power off.

Vs1 is the stalling speed at maximum all-up weight in the clean configuration.

Vfe is the maximum flap extension speed.

Vno is the maximum structural cruising speed.

Va is the design manoeuvring speed. This speed allows for gusty or turbulent conditions, such that the load factor does not exceed safe limits.

Vle is the maximum speed for the landing gear extended.

Vlo is the maximum speed for the actual operation of the landing gear.

Vyse is the single-engine best rate of climb.

Vx is the best angle of climb.

Vne is the never-exceed speed.

Vat is the threshold speed to be achieved before touch-down.

These speeds are all indicated airspeed, but what’s that?

Indicated Airspeed (IAS)

This is the airspeed that is actually displayed on the instrument. Normally, the ASI dial is colour-coded: yellow denotes the caution range; green the normal range; and white the flap limiting speed. At the end of the green segment is the maximum normal operating speed, or Vno. Vne is marked by a red line, and this speed should never be exceeded.

Calibrated Airspeed (CAS)

When indicated airspeed is corrected for instrument error, or errors in manufacture or calibration of the ASI, it is known as calibrated airspeed. The corrections that should be applied are normally very small and can be found in the pilot’s operating handbook.

Rectified Airspeed (RAS)