ATA 31A Instruments

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Aviat Aviation ion Colleg ollege

Traini raining ng Manua anuall Fundamentals J et Aircraft Maintenance Fundamentals ATA 31 A Instruments J AR-66 AR-66

Boo Book No: No:

J AMF AMF AT ATA 31 31 A ALL ALL

Lufthansa Tec Tech hnical ical Trai Train ning ing Gmb GmbH Lufthan Lufthansa sa Base

Issue: Februa February ry 2002 2002 For Train Trainin ing g Purposes Purposes Only Lufthansa Lufthansa 2002 2002

For train rainin ing g purp purpose ose and and int interna ernall use only only. Copyri Copyrigh ghtt by Luftha Lufthansa nsa Techni echnical cal Train Trainin ing g GmbH. GmbH. All righ rightts reserv reserved. ed. No part parts s of this his train trainin ing g manu anual may be sold sold or repr eprodu oduced ced in any any for form withou ithoutt perm permission of:

Lufthansa hansa Technical chnical Trainin raining g GmbH GmbH Luftha Lufthansa nsa Base Frank Frankfur furtt D-60546 D-60546 F rankfur rankfurt/ t/Main Main Tel Tel.. +49 69 / 696 41 78 Fax Fax +49 69 / 696 63 84 Luftha Lufthansa nsa Base Ham Hamburg burg Weg beim eim J ger ger 193 193 D-22335 D-22335 Hambur Hamburg g Tel Tel.. +49 40 / 5070 24 13 Fax Fax +49 40 / 5070 47 46

For train rainin ing g purp purpose ose and and int interna ernall use only only. Copyri Copyrigh ghtt by Luftha Lufthansa nsa Techni echnical cal Train Trainin ing g GmbH. GmbH. All righ rightts reserv reserved. ed. No part parts s of this his train trainin ing g manu anual may be sold sold or repr eprodu oduced ced in any any for form withou ithoutt perm permission of:

Lufthansa hansa Technical chnical Trainin raining g GmbH GmbH Luftha Lufthansa nsa Base Frank Frankfur furtt D-60546 D-60546 F rankfur rankfurt/ t/Main Main Tel Tel.. +49 69 / 696 41 78 Fax Fax +49 69 / 696 63 84 Luftha Lufthansa nsa Base Ham Hamburg burg Weg beim eim J ger ger 193 193 D-22335 D-22335 Hambur Hamburg g Tel Tel.. +49 40 / 5070 24 13 Fax Fax +49 40 / 5070 47 46

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INSTRUMENTS INTRODUCTION

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INSTRUMENTS CLASSIFICATION In large commercial aircraft you can find many instruments. They are used to monitor and control the flight, the engines and the aircraft systems. The flight instruments provide all data to perform a safe an economic flight. They are located on captains instrument panel and are repeated on copilots instrument panel. The engine indications are located on the center instrument panel and the airframe system indications for the electric, hydraulic, fuel and other systems, is usually found on the overhead panel or flight engineer panel. Information about the engine indications can be found in unit 77 of the J AMF course and system indications are in specific units, like unit 28 for the fuel system. The instruments which are used to control and monitor the flight can be divided further, into the navigation instruments which you will see in unit 34 and the flight instruments which are subject of this unit.

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FUNDAMENTALS ATA 31 A Lesson 1



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Figure 1

Instruments Classification





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BASIC T The flight instruments on the captains and copilots instrument panels have an identical layout. The most important instruments to fly the aircraft are located in the center in the so called basic T arrangement. The altimeter indicates the altitude of the aircraft above sea level or the airport. You will learn about it in detail in lesson 2 of this unit. The airspeed indicator shows a speed which is only identical to the real airspeed of the aircraft when you fly at sea level, but it is a reference for all aerodynamic parameters influencing aircraft lift, drag and maneuverability. You will learn more about it in lesson 2 of this unit. The compass or heading indication is integrated in the Horizontal situation indicator, or HSI in short. It shows the direction of the aircraft longitudinal axis in relation to magnetic north. You will learn more about it in lesson 8 of this unit. Other indication in the HSI are discussed in the navigation unit 34. Additional flight instruments which you will learn about in this unit are the Vertical speed indicator, the Mach meter and the temperature indicators. The photograph on the right side of the screen shows the instrument panels of a modern glass cockpit aircraft, in this example, an A340. You will also find the same indications in this cockpit design, but it is discussed in more detail in the Electronic Instrument Systems part of the course. Even in small aircraft you can find the same arrangement of the flight instruments.

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Figure 2

Basic T Instruments

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INSTRUMENTS AIR DATA INSTRUMENTS

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AIR DATA INSTRUMENTS INTERNATIONAL STANDARD ATMOSPHERE In this lesson we will show you the air data instruments, such as the altimeter and airspeed indicator. These instruments use air pressure information from outside the aircraft. To understand their function you must be familiar with the international standard atmoshere, ISA in short, and what pressures act on the aircraft during flight. You can find this information in the aerodynamic unit.

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Figure 3

ISA Standard Atmosphere





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ALTIMETER FUNCTION We will begin this Air data instruments lesson with a closer look at the Altimeter. In an aircraft you can always find the Altimeter in the same location of the basic--T even in small aircraft. The altimeter is a very important indicator because it shows the pilot if the aircraft is flying at the required altitude. You can find various types of altimeters in aircraft. In small piston engine aircraft the Altimeter has two Pointers, one for 100 ft and the second for 1000 ft. The altimeters in aircraft which can fly at higher altitudes has either an additional pointer for the indication of 10000 ft or it uses a digital readout for the total altitude and just one pointer for the analog indication of the one hundred feet range. This type is most common in larger aircraft with conventional instruments.

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Figure 4

Different Altimeter Types





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altimeter function cont. All altimeters have the same basic function: They simply measure the static pressure of the atmosphere and indicate it in feet or meters. You know that the static pressure at sea level is 1013.25 hPa, which corresponds to 1013.25 mb of the old unit. To make it easier we will use just 1013 hPa in the following discussions. With increasing altitude the pressure decreases. At 18000 ft the pressure is only 50% of the pressure at sea level and at 36000 ft the pressure has decreased to 25%. At 54000 ft the pressure is again halved to just 12.5% of the value at sea level. A pneumatic altimeter, which you can find in small aircraft and also as a standby altimeter in larger aircraft, has a sensitive pressure gauge, called aneroid bellows. The static pressure which is applied to the sealed case surrounds the bellows and compresses it. The bellows deflection moves the instrument mechanism and turns the altitude pointer which shows the altitude on a scale calibrated in feet or meters. The altimeter indication is calibrated to the standard atmosphere shown with the graph on the left. Please note that near sea level a certain pressure difference only changes the altitude by a small amount but at higher altitudes the same pressure difference has a larger effect on the altitude. This means that accuracy of altimeters decrease with the altitude. The main altimeters of large aircraf don#t have bellows. They are driven by an air data computer and use only electrical components such as motors, amplifiers or even digital computers.

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Figure 5

Altimeter Basic Function





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ALTIMETER SETTINGS You have learned in the last segment that the altimeter is calibrated to the conditions of the standard atmospere. A closer look at the pressure graph near sea level should make this clear again. You can see that the graph is nearly linear in this area and that the altitude increases by 30 ft for each pressure decrease of 1 hPa. When an aircraft flies at an altitude of 1000 ft, the outside static pressure is about 980 hPa The altimeter converts this static pressure to an indication of 1000 ft. The indicated altitude is therefore equal to the true altitude. As you can imagine, this scenario is only correct if the pressure at sea level is really 1013 hPa This weather map shows you that the real pressure at sea level changes continously and is only 1013 hPa at certain locations for a certain time, here for example in Hamburg. Other areas have low pressure or high pressure. When you fly fromHamburg to Lulea in northern Sweden, the altimeter readings are always correct because the pressure corresponds to standard conditions.

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Figure 6

Altimeter Settings





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altimeter settings cont. Due to the actual weather conditions the pressure in London is 1000 hPa at sea level. To indicate one 1000 ft the altimeter needs a static pressure of about 980 hPa. This means that the true altitude will be about 600 ft when you arrive in London. To solve this problem with different pressures at sea level each altimeter has a baro set knob. With the baro set knob on the instrument you give the altimeter a new reference pressure. In pneumatic indicators the knob directly turns the whole altimeter mechanism to set a new reference value. Electrical indicators do the correction electrically inside the indicator or via the air data computer. When you select 1000 hPa with the baro set knob, the altimeter will show 600 ft, because the difference between the reference and the actual pressure is 20 hPa

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Figure 7

Altimeter Settings





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altimeter settings cont. Now lets assume that the aircraft lands at London Heathrow which has a field elevation of 80 ft. The altimeter indicates the field elevation, in this example 80 ft in London, if you set a baro reference which corresponds to the pressure at sea level. This baro setting is called QNH and is the normal setting during takeoff and landing. You get an indication of 0 ft if you set the baro knob to the actual pressure of the airport. This setting is called QFE and can be used instead of QNH for takeoff and landing.

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Figure 8

Altimeter Settings (QNH+QFE)





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altimeter settings cont. You can#t compare altitude indications when you use different baro settings. This means that an altitude separation by ATC is not possible. Therefore it is very important that all altimeters use the same baro setting during cruise flight. This standard setting of 1013 hPa must be selected during climb at a certain altitude, called the transition altitude. Now ATC can clear all aircraft to different altitudes to separate them vertically. This altitude is now called flight level which is calculated by dividing the altitude by one hundred. During descent at the so called transition level the setting is changed back to the QNH or QFE of the destination to get the correct altitude indications during landing.

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ATA 31A Instruments

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