ppl_06_e2

ID: 3658

Customer: Oleg Ostapenko E-mail: [e-mail]

Customer: Oleg Ostapenko E-mail: [e-mail]

CH AP T ER 1 4 : P R ESSU R E INST R

ALTIMETRY DEFINITIONS.

There follow several common definitions used in the subject of altimetry.

T r a n s i t i o n Al t i t u d e .

Figure 14.18 defines the differences between Transition Altitude, Level and Layer The Transition Altitude is the altitude at, or below which, the vertical position of an aircraft is expressed and controlled in terms of altitude. An altimeter reads altitude when the correct QNH or Regional Pressure Setting is set in the altimeter subscale.

Figure 14.18 Transition Altitude, Transition Layer and Transition Level.

In the United Kingdom, the transition altitude is 3 000 feet except in or beneath airspace specified in the UK AIP, for example, in the London TMA, the transition altitude is 6000 feet.

T r a n s i t i o n Le v e l .

The Transition Level is the lowest Flight Level available for use above the Transition Altitude. At and above Transition Level, vertical position is expressed as a Flight Level. The altimeter reads Flight Level when the Standard Pressure Setting (SPS) of 1013.25 hectopascals (millibars) is set in the altimeter subscale.

T r a n s i t i o n La y e r .

This is the space between Transition Altitude and Transition Level. When climbing through Transition Layer, the aircraft’s vertical position is expressed in terms of Flight Level; when descending through the Transition Layer the aircraft’s vertical position is expressed in terms of altitude. Pilots must not assume that separation exists between the Transition Altitude and the Transition Level.

F l i g h t Le v e l s .

Flight Levels are surfaces of constant pressure related to the SPS of 1013.25 hectopascals (millibars). Flight Levels are separated by specified pressure intervals. In the United Kingdom, these intervals are 500 feet between Transition Level and Flight Level 245, while from Flight Level 250 upwards the intervals are 1 000 feet. A Flight Level is expressed as the number of hundreds of feet which would be indicated, at the level concerned, by an ISA-calibrated altimeter on which the subscale is set to 1013.25 millibars, or 29.92 inches of mercury. For example, at 4 500 feet the Flight Level would be FL 45.

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Customer: Oleg Ostapenko E-mail: [e-mail]

Customer: Oleg Ostapenko E-mail: [e-mail]

CH AP T ER 1 4 : P R ESSU R E INST R U MENT S

ALTIMETER SETTINGS.

It is important to understand that the altimeter indicates vertical distance above the pressure level set on its subscale. In the UK, pressure settings are expressed in millibars(mb). The unit commonly used throughout Europe is the hectopascal (hPa). The mb and the hPa are identical in value. The hPa is the standard JAA unit. There are four altimeter pressure settings: QFE, QNH, Regional Pressure Setting (RPS) and the Standard Pressure Setting (SPS): 1013.25 hPa (mb).

Q F E.

Figure 14.19 With QFE set, altimeter reads height.

QFE is the aerodrome level pressure which, when set on the altimeter subscale, will cause the altimeter of an aircraft on the ground to read zero, assuming there is no instrument error.

In flight, with QFE set, the altimeter will indicate height above the aerodrome QFE reference datum, provided ISA conditions prevail between aerodrome level and the aircraft, and that there are no other altimeter errors. In practice, QFE is used mainly for circuit-flying and for flight in the immediate vicinity of a pilot’s home aerodrome.

Q NH .

Figure 14.20 With QNH set, altimeter reads altitude.

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ID: 3658

Customer: Oleg Ostapenko E-mail: [e-mail]

Customer: Oleg Ostapenko E-mail: [e-mail]

CH AP T ER 1 4 : P R ESSU R E INST R

Aerodrome QNH is the observed aerodrome pressure converted to the pressure of

Mean Sea Level in accordance with the ICAO Standard Atmosphere.

With aerodrome QNH set on the subscale, the altimeter of an aircraft on the ground at the aerodrome indicates elevation, that is, the height of that part of the aerodrome above Mean Sea Level.

With QNH set, the altimeter of an aircraft, in flight, will indicate altitude, that is, vertical distance above Mean Sea Level.

R e g i o n a l P r e s s u r e Se t t i n g ( o r F o r e c a s t Q NH ) .

With QNH set, the altimeter reads the vertical distance of the aircraft above Mean Sea Level in the local area. Because the number of aerodromes reporting actual QNH is limited, the UK is divided up into Altimeter Setting Regions, (see Figure 14.26, p220).

The Met Office forecasts the lowest QNH for each of these regions every hour, and sends these Regional Pressure Setting forecasts to Air Traffic Service Units. When flying cross-country below the Transition Altitude, a pilot will normally set the Regional Pressure Setting for the area he is transiting, for example, Cotswold Regional Pressure Setting.

Figure 14.21 Altimeter Reading with SPS Set.

St a n d a r d P r e s s u r e Se t t i n g ( SP S) .

When the Standard Pressure Setting of 1013.25 hectopascals or millibars is set on the subscale, the altimeter shows the aircraft’s vertical separation from that pressure level wherever it may lie. This setting is used in the United Kingdom above the Transition Altitude or Transition Level. An aircraft’s vertical separation from the 1013.25 hectopascal pressure level is known as its “pressure altitude”. The “pressure altitude” of an aircraft is expressed in terms of Flight Levels as already detailed on page 213. With all aircraft above the Transition Level having their altimeter subscales set to 1013, air traffic controllers can ensure safe vertical separation between aircraft under their control.

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Customer: Oleg Ostapenko E-mail: [e-mail]

Customer: Oleg Ostapenko E-mail: [e-mail]

CH AP T ER 1 4 : P R ESSU R E INST R U MENT S

ALTIMETER ERRORS.

T i m e La g .

With many types of altimeter, the response to change of altitude is not instantaneous. This causes the altimeter to under-read in a climb and over-read in a descent. The lag is most noticeable when the change in altitude is rapid and prolonged.

When flying

in air which is warmer

than the ICAO Standard Atmosphere, the altimeter will under-read.

If a static vent became

blocked during level

flight in icing conditions and the aircraft subsequently climbed, the ASI would under-read and the readings on the Altimeter and the VSI would remain unchanged.

If an unpressurised

aircraft has an alternate

static source within the cockpit, the alternate static pressure will be lower than the outside static source.

In s t r u m e n t Er r o r .

Manufacturing imperfections, including friction in the linkage, cause errors throughout the operating range. The errors are kept as small as possible by adjustments within the instrument, and the calibration procedure ensures that they are within permitted

tolerances. Residual errors may be listed on a correction card.

P o s i t i o n ( o r P r e s s u r e ) Er r o r .

Position error is largely due to the inability of the altimeter to sense the true static pressure outside the aircraft. The error is usually small. Position Error is covered on Page 203.

Ma n o e u v r e - i n d u c e d Er r o r .

Manoeuvre-induced error is caused mainly by transient fluctuations of pressure at the static vent during change of pitch attitude.

T e m p e r a t u r e Er r o r .

Even with no other errors at all, the pressure altimeter will not indicate the true altitude unless the surface temperature and lapse rate of the column of air are those which were assumed in the calibration of the altimeter, (i.e. ISA conditions).

When flying in air which is colder than the ISA , the altimeter will over-read.

When flying in air which is warmer than the ISA, the altimeter will under-read.

Where the temperature at cruising level deviates from ISA assumptions, an approximate correction can be made with most navigational computers. The correction will, however, only be approximate since vertical temperature variations

are not known.

De n s i t y Al t i t u d e .

Density altitude can be defined as the altitude in the ICAO Standard Atmosphere at which the density prevailing at the location of measurement would occur. It is a convenient parameter by which to measure engine performance.

Density altitude can be obtained by use of an airspeed correction chart or by navigational computer. If the prevailing density decreases, the location at which the density is measured will correspond to a greater altitude in ISA. Thus, density altitude increases.

Ba r o m e t r i c Er r o r .

Providing the altimeter has a pressure sub-scale, and the local QNH is set on it, the altimeter will indicate height above sea level. If the local surface pressure changes,

it will result in the altimeter over-reading or under-reading.

Bl o c k a g e s An d Le a k s .

If the static source becomes blocked, the altimeter will not register any change in altitude. In the case of a static source blockage, the altitude at which the blockage occurred will remain the indicated altitude, regardless of any climb or descent. A

216

ID: 3658

Customer: Oleg Ostapenko E-mail: [e-mail]

Customer: Oleg Ostapenko E-mail: [e-mail]

CH AP T ER 1 4 : P R ESSU R E INST R

Figure 14.22 A VSI-indicated level flight.

fracture or leak in the static line within the cockpit of an unpressurised aircraft will normally result in the altimeter over-reading. The pressure in the cockpit will be lower than ambient pressure because of aerodynamic suction.

Ch e c k s Be f o r e T a k e - o f f .

The following checks of the altimeter should be made before flight.

The instrument dial glass must be clean and undamaged and the correct altimeter setting must be set on the subscale.

If you are operating from a controlled aerodrome, Air Traffic Control will pass you the correct altimeter setting, normally aerodrome QNH. With the QNH set, the altimeter will display the aerodrome elevation. With the QFE set, the altimeter should read zero.

THE VERTICAL SPEED INDICATOR.

The Vertical Speed Indicator (VSI) senses static pressure, like the altimeter but is so constructed that it indicates rate of climb or descent in hundreds, or thousands of feet per minute.

P r i n c i p l e o f O p e r a t i o n o f t h e V SI.

The construction of the VSI is shown in Figure 14.22. When an aircraft departs from level flight, the static pressure acting on an aircraft changes accordingly. The VSI measures the pressure difference between each side of a metering unit within the instrument case. In level flight, the pressures on each side of the metering unit are the same, (see Figure 14.22). But whereas, during a climb or descent, the static vent immediately senses the change of atmospheric pressure, the static pressure in the VSI instrument case changes at a lower rate because of the presence of the metering unit. The difference in pressure across the metering unit will last throughout a climb or descent, causing a rate of climb or descent to be indicated. Once the aircraft is in level flight again, the pressure across the metering unit equalises and the VSI indicates 0; that is, level flight.

T h e Co n s t r u c t i o n o f t h e V SI.

As is shown in Figure 14.22, static pressure from the static vent is fed to a capsule located within the airtight case of the VSI instrument. The instrument case itself is also fed with static pressure from the same vent, but this feed comes through a restricted metering unit. Thus as the static pressure changes, the pressure within the

The principle

of operation of the vertical

speed

indicator is that it compares static pressure, sensed through a direct static vent, with that in the instrument’s case, sensed through a metered vent.

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Customer: Oleg Ostapenko E-mail: [e-mail]

CH AP T ER 1 4 : P R ESSU R E INST R U MENT S

case surrounding the capsule changes at a slower rate than that within the capsule. For example, if the aircraft is climbing, because of the action of the metering unit the pressure in the capsule will be less than that in the VSI instrument case. The capsule will, therefore, be compressed, this compression being converted by a suitable linkage to a pointer which indicates the rate of climb. On the other hand, if the aircraft is descending, the pressure in the case will be less than the pressure in the capsule, causing the capsule to expand. This expansion will move the linkage so that the pointer indicates the rate of descent on the VSI instrument face.

V SI P r e s e n t a t i o n .

Two types of presentation are available: a linear scale and a logarithmic scale. It will be obvious from comparing the two instruments depicted in Figure 14.23, that the logarithmic scale, shown on the right, is easier to read at the lower rates of climb and descent.

Figure 14.23 Presentation of the VSI.

THE ERRORS OF THE VSI.

In s t r u m e n t Er r o r .

Instrument error is due to manufacturing imperfections.

P o s i t i o n ( o r P r e s s u r e ) Er r o r .

If the sensing of static pressure changes is subject to position error, the Vertical Speed Indicator will wrongly indicate a climb or descent when speed is suddenly changed. This error is most noticeable during take-off acceleration.

Ma n o e u v r e - i n d u c e d Er r o r .

Any fluctuations in pressure at the static vent during attitude changes will cause the instrument to indicate a false rate of climb or descent. Consequently, most VSIs have a small counterbalance weight included in the linkage, the inertia of which causes delays in the indications of changes in vertical speed during manoeuvres.

T i m e La g .

The VSI pointer takes a few seconds to steady because of the time taken to build up a steady pressure difference during climb or descent.

There will also be a time lag when the aircraft transits to level flight because of the time taken for the pressures to equalise. This error is most noticeable after a prolonged climb or descent, especially at a high rate.

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ID: 3658

Customer: Oleg Ostapenko E-mail: [e-mail]

Customer: Oleg Ostapenko E-mail: [e-mail]

CH AP T ER 1 4 : P R ESSU R E INST R

T h e In s t a n t a n e o u s V SI

To overcome the problem of lag, the instantaneous, or IVSI, incorporates an accelerometer unit which responds quickly to a change of altitude. In a descent, as shown in Figure 14.24, the piston in the vertical acceleration pump immediately rises in the cylinder and increases the pressure in the capsule.

Figure 14.24 ISVI Mechanism showing Descent.

The capsule therefore, expands almost immediately and the pointer will give an instant indication of descent. If the vertical acceleration ceases, after a few seconds the piston will slowly descend to its original position, but by this time the correct differential pressure between the capsule and the case will have been set up and the correct rate of descent will continue to be shown.

Figure 14.25 The Mechanism in a Climb.

In a climb, as shown in Figure 14.25, the piston in the vertical acceleration pump immediately falls in the cylinder and decreases the pressure in the capsule. The capsule contracts and the pointer will give an instant indication of climb. If the vertical acceleration ceases, after a few seconds the piston will slowly rise to its original position. But by this time the correct differential pressure between the capsule and the case will have established itself and the correct rate of climb will continue to be shown.

If the static

vent leading to the Vertical

Speed

Indicator becomes blocked while an aircraft is descending, the indicator will return to

zero after a short delay.

Bl o c k a g e s a n d Le a k s .

Any blockages of the static line or vent will cause the needle to return to zero after a short delay. If the supply of air to the VSI is blocked, it is highly probable that the other pressure instruments, the Air Speed Indicator and the Altimeter, will also be affected.

Ch e c k s Be f o r e T a k e - o f f .

The following checks of the vertical speed indicator should be made before flight.

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Customer: Oleg Ostapenko E-mail: [e-mail]

CH AP T ER 1 4 : P R ESSU R E INST R U MENT S

•The instrument dial glass must be clean and undamaged and the instrument should read zero.

•In flight, the accuracy of the VSI may be checked against the altimeter and a stop watch during a steady climb or descent.

•The VSI should indicate zero when in level flight.

Figure 14.26 UK Regional Pressure Setting (RPS) Areas.

220

ID: 3658

Customer: Oleg Ostapenko E-mail: [e-mail]

Customer: Oleg Ostapenko E-mail: [e-mail]

CH AP T ER 1 4 : P R ESSU R E INST R U MENT S

R e p r e s e n t a t i v e P P L - t y p e q u e s t i o n s t o t e s t y o u o f P r e s s u r e In s t r u m e n t s .

1.If, while an aircraft is descending, the static vent leading to the Vertical Speed Indicator becomes blocked, the indicator will:

a.continue to show the same reading

b.indicate a climb

c.indicate a descent

d.show a zero reading, after a short delay

2.When an aircraft is in flight, the pressure sensed by the forward facing hole in the pitot tube is:

a.static pressure only

b.total pressure plus dynamic pressure

c.dynamic pressure plus static pressure

d.dynamic pressure only

3.The principle of operation of the Vertical Speed Indicator (VSI) is that it:

a.compares static pressure in a capsule, sensed through a direct static vent, with that in the instrument’s case, sensed though a metered vent. The VSI is, thus, able to detect the rate of change of static pressure with height

b.compares total pressure from the pitot tube with static pressure from the static vents. The VSI is calibrated to show the difference between the two as a vertical speed in feet per minute

c.compares dynamic pressure from the pitot tube with static pressure from the static vents. The VSI is calibrated to show the difference between the two as a vertical speed in feet per minute or metres per second

d.senses total pressure only, from the pitot tube, and converts the change in total pressure with height into a rate of climb or descent, measured either in feet per minute or metres per second

4.If, during descent, the static sources to the Airspeed Indicator and Altimeter become blocked by ice:

a.the Airspeed Indicator will over-read and the Altimeter will underread

b.the Airspeed Indicator will under-read and the Altimeter will overread

c.both instruments will over-read

d.both instruments will under-read

5.Ignoring any instrument or position errors, in what conditions will the Airspeed Indicator indicate the True Airspeed of an aircraft?

a.at any altitude or temperature

b.in ICAO Standard Atmosphere, sea-level conditions only

c.at any altitude, provided that the temperature lapse rate is in accordance with ISA

d.at any altitude, but only when ISA conditions prevail

221

Order: 6026

Customer: Oleg Ostapenko E-mail: [e-mail]

Customer: Oleg Ostapenko E-mail: [e-mail]

CH AP T ER 1 4 : P R ESSU R E INST R U MENT S Q U EST IO NS

6.If an unpressurised aircraft has an alternate static source within the cockpit, the alternate static pressure:

a.must be selected immediately there is any fluctuation of the Airspeed Indicator

b.will be higher than the outside static source

c.will be unreliable because of ingress of moisture from the pitot head

d.will be lower than the outside static source

7.An Altimeter:

a.contains a barometric capsule, connected to a total pressure source, that contracts during a descent

b.contains an aneroid capsule connected to a static pressure source. The capsule contracts during a descent

c.contains a barometric capsule that expands during a descent

d.contains a partially evacuated capsule that expands during a descent

8.If a static vent became blocked during level flight in icing conditions and the aircraft subsequently climbed, the readings on the altimeter, the VSI and the ASI would:

	Altimeter	VSI	ASI
a.	remain unchanged	remain unchanged	under-read
b.	remain unchanged	under-read	over-read
c.	under-read	remain unchanged	over-read
d.	over-read	over-read	under-read

9.If the power supply to the pitot heater failed during flight in icing conditions and the aircraft subsequently descended, the readings on the Altimeter, the VSI and the ASI would, if ice had blocked the pitot (total pressure) tube:

Altimeter

VSI

ASI

a.

read correctly

under-read

over-read

b.

under-read

read correctly

over-read

c.

read correctly

read correctly

over-read

d.

read correctly

read correctly

under-read

Question

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Answer

T h e a n s w e r s t o t h e s e q u e s t i o n s c a n b e f o

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