LESSON: Aircraft Flight Instruments

5-A 2 pitot/static and vacuum systems.doc

LESSON: Aircraft Flight Instruments

OBJECTIVE:

To develop the students understanding of the pitot-static/vacuum systems and associated instruments (BE-76 specific)

ELEMENTS:

Elements of pitot-static/vacuum systems and associated instruments

·  Pitot Static System

·  Altimeter

·  Airspeed Indicator

·  Vertical Speed Indicator

·  Vacuum System

·  Attitude Indicator

·  Heading Indicator

STUDENTS ACTIONS:

·  Listen, take notes, ask questions

·  Respond to instructors questions

·  Leave with a framework understanding of the subject

INSTRUCTORS ACTIONS

·  Discuss lesson objective

·  Introduce the various flight instruments

·  Discuss each of the pitot-static/vacuum components individually

·  Describe their operation and use

·  Ask pertinent questions to determine students understanding

·  Assign appropriate study material

COMPLETEION STANDARDS:

The student should demonstrate adequate understanding of the pitot-static/vacuum systems and associated instruments by:

successfully completing an oral quiz or written exam

Introduction

Attention/Motivation

The flight instruments are an integral part of attitude instrument flying. Therefore an understanding of these instruments is essential for interpreting them and detecting errors

Overview

This lesson will discuss elements of, aircraft flight instruments and their operating methods and characteristics

DEVELOPMENT

Pitot Static System & Instruments

·  Pitot static instruments: Airspeed indicator, Altimeter, and Vertical speed indicator

·  A pitot tube for the pilot’s flight instruments is mounted on the outboard lower left wing provides ram air pressure for the airspeed indicator. An optional pitot tube for copilot’s flight instruments is located on the outboard lower right wing

·  The Pitot tube is heated, through a heating element

·  Pitot heat is activated by an on/off rocker switch and protected by a 10 Amp circuit breaker

·  Two static ports located on the left and right sides of the aft fuselage, provide static air pressure for the vertical speed indicator and altimeter

·  A alternate static air pressure source is mounted located on the left sidewall

·  It is activated by a lever on the lower sidewall (OFF NORMAL – ON ALTERNATE)

·  Activation will cause a momentary raise in airspeed, altitude and vertical speed because of the lower pressure in the cabin

·  Moisture in the static air plumbing is drained by placing the alternate static source lever in the ON ALTERNATE position momentarily and then returning it to the OFF NORMAL position

Airspeed Indicator

·  Simply a specialized air pressure gauge, that measures dynamic air pressure

·  Ram air pressure moves a diaphragm which is connected by linkages to the airspeed needle

·  Displays KIAS

·  Limitations and range markings include:

1.  White Arc (Flap operating range) 42 (Vso) to 100 knots

2.  Green Arc (Normal operating range) 50 (Vs1) to 145 knots

3.  Yellow arc (Caution range) 145 (Vno) – 164 knots

4.  Red line (Never exceed speed) 164 knots (Vne)

·  Airspeed indicator suffers from position error at high angles of attack and in slips/skids

·  IAS – Indicated airspeed: Airspeed read off the guage

·  CAS – Calibrated airspeed: IAS corrected for instrument and position error

·  EAS – Equivalent Airspeed: CAS corrected for the compression of air at a particular altitude. Normally not considered significant when operating below 200 knots

·  TAS – True airspeed: CAS corrected for temperature and pressure

Altimeter

·  The altimeter is an aneroid barometer

·  Opening on one side to allow static pressure to enter a sealed case

·  A series of sealed diaphragms “aneroid disks” expand and contract in response to the changing static pressure: up – out, down – in

·  These disks are mechanically linked to the altitude needle

·  Altimeter is equipped with a calibration knob used to set the altimeter to the current barometric pressure

·  When the altimeter is set to the current altimeter setting it should be within +/- 75 feet of station elevation

·  Indicated altitude – the altitude as shown on the altimeter

·  True altitude – Actual height above sea level

·  Absolute altitude – Actual height above the terrain below

·  Pressure altitude – Altitude shown on the altimeter when set to 29.92 (standard)

·  Altimeter must be adjusted for pressure changes and changes

·  For safety sake true altitude should equal indicated altitude

·  If the airplane enters lower pressure and the altimeter is not adjusted it will read lower than actual

·  If the airplane enters lower pressure and the altimeter is not adjusted it will read higher than actual

·  “High to low, look out below. Low to high you’re in the sky”

·  Density altitude is an important altitude, not shown on the altimeter

·  Density altitude is pressure altitude corrected for nonstandard temperature

·  Affects aircraft performance

Vertical Speed Indicator

·  Not a required instrument under IFR, but very helpful

·  Static air pressure enters a chamber with a calibrated leak

·  Air pressure flows in or out, moving a diaphragm connected by mechanical linkages to an indicator needle

·  Calibrated to read ascents or descents in feet per minute

Pitot static system errors

·  Errors in a pitot static instrument is usually caused by a blockage in the plumbing

·  Pitot tube blockage affects only the airspeed indicator

·  If the Pitot tube is blocked but the drain hole remains open the AS will read zero

·  If the Pitot tube and drain hole become clogged the airspeed indicator will act like an altimeter: airplane climbs airspeed increases

·  Static port blockage affects all pitot static instruments

·  Altimeter and VSI will not move

·  Airspeed indicator will still react normally to changes in airspeed, however the actual airspeed displayed will be incorrect. Above the altitude where the port became clogged airspeed will read lower than it should

·  If the static port should become blocked an alternate static source is available

·  This alternate static source is located in the cockpit

·  The pressure inside the cabin is lower than the outside air due to slipstream

·  Altimeter will read a little higher than actual

·  Airspeed will read a little faster than actual

·  Vertical speed indicator will show a momentary climb

·  If the airplane is not equipped with an alternate static source you can break the glass of the VSI to create one

Gyroscopic Flight Instruments

·  Attitude Indicator and Heading Indicator operate by an engine driven vacuum pump

·  Turn coordinator is electrically driven

·  Gyroscopic instruments operate on the rigidity in space principal that applies to all spinning objects

·  Vacuum system consists of:

1.  2 engine driven, dry, pressure pumps interconnected to form a single system

2.  Vacuum check valves

3.  Vacuum air filter

4.  Suction gauge & low vacuum lights: Normal range 4.5 – 5.4 InHg

Attitude Indicator

·  Also called the Artificial Horizon

·  Gives a visual picture of the flight attitude, substitute for the actual horizon

·  Fixed Miniature airplane, and movable horizon reference line

·  Any adjustments in pitch or bank attitude are made in reference to the AI

·  At the heart of the AI is a self erecting gyroscope with a vertical spin axis

·  Subject to gyroscopic precession errors, greatest when rolling out of a steep bank 180° turn

·  Also subject to acceleration errors

·  The AI should self-correct for Precession and Acceleration errors fairly quickly

·  Preflight Check:

1.  Must stabilize within 5 minutes of startup

2.  During turns on the ground must not display a bank angle of more than 5°

Heading Indicator

·  Compass card rotates around a fixed airplane image to display heading

·  Unless the HI is slaved it must be aligned with the magnetic compass to display the proper heading

·  Heading indicator is subject to gyroscopic precession

·  Must be reset according to the magnetic compass occasionally (every 15 minutes) because of mechanical reasons and apparent drift

·  Preflight Check:

1.  During turns on the ground should show correct turn direction: Turning right heading increases, turning left heading decreases

2.  While on the runway prior to initiating the takeoff the heading indicator should be checked to ensure it is displaying the runway MH

Turn Coordinator

·  Main purpose is to allow the pilot to enter and maintain standard rate turns

·  Standard rate turn is 3° per second, so a 360° turn would take 2 minutes

·  Turn coordinator is electrically driven

·  The gyroscope is set at a 30° angle to the airplane longitudinal axis, this allows the instrument to sense rolling as well as turning

·  Miniature airplane displays the direction of bank and rate of turn

·  Inclinometer on the bottom indicates to quality of the turn

CONCLUSION

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