Basic Instruments

Basic
Instruments

DIC/DRC (E-type) - CH 11

Acceleration Errors

NANDS (no error N/S)

Turning Errors

NUNOS (no error E/W)

Swirl Effect (goes in direction of centre of turn

opposite of U or O
+ve for U
-ve for O

SHA
Sensitivity

Horizontallity

Aperiodicity

Lubber line
Compass Card (pendulously suspended on iridium pivot)
Damping oil (SYLPHON TUBE for expansion, low viscosity)

Residual Dip ( to nearest pole AND CG moves opposite)
pivot is opposite side to CG!!!

Whenever a compass heading lags an AC we call it SLUGGISH otherwise it’s LIVELY

- $Compass Error = Latitude + \frac{Bank Angle}{2}$

Gyros

Spin axis is NOT Counted as a DoF!!!

Drift to the Right always in the NH (see hand out) - the heading will decrease

Generally Electrically driven Gyros are smaller = more compact instrument

Gyro Basics - Ch 13

Earth rotates EASTWARDS at @ 600kts

That is precisely 360/23.934 = 15.04 deg per hr (at the poles)

ER Varies with the SINE of the LATITUDE for a Directional Gyro

Wander

Topple: motion in the vertical plane is called topple.
Drift: motion in the horizontal plane is called drift.
NB: A gyro with only a vertical axis can TOPPLE whereas a gyro with horizontal axis can TOPPLE and DRIFT
Real Wander - mechanical imperfections
Apparent Wander = ER and TW

Rigidity

Rotational speed
Mass
Mass distribution

REST - TAXD

Transport Wander and ER effect are cumulative
At Poles
Vertical Gyros appear NOT TO DRIFT or TOPPLE

Horizontal gyros appear to Drift
At the Equator
Vertical Gyros appear to Topple

An ‘East Aligned’ Horizontal Gyro appears to topple backwards to the vertical

A ‘North Aligned’ Horizontal Gyro will not Topple or Drift if it remains at the Equator
SD_TC: Apparent Drift varies with the SINE of the latitude. Apparent Topple varies with the COSINE of the latitude.

Directional Gyro (DG) - Ch 14

DG is a horizontally tied displacement gyro
Not magnetic
Compares AC longitudinal axis to Gyro-rotor alignment
CAGING - done by pilot
DG must be re-synched with Compass every 10mins

Or after heavy manoeuvres
REST - TAXD

Maintaining Gyro Erection

Total Drift = ER + TD + RD + LN

ER (Earth rate) (-ve NH, +ve SH)
ER = -15.04 x sine aircraft latitude (Northern hemisphere) (positive for southern)

LN = opposite sign to ER i.e. in NH +ve
TD (Transport Drift)
RD (Random Drift i.e mechanical imperfections)
LN (Latitude Nut)
- LN DONE BY ENGINEER at you location, uses the principle of precession
  The LN adjustment is used to OPPOSE the ER effect and therefore have an opposite sign mathematically
If Q states LOCAL FLYING or HEADING ‘DUE NORTH’ Then TD = ZERO
If Q states ‘MECHANICALLY PERFECT’ then RD = ZERO

In NH:

Indicated heading will always reduce
Compass/Gyro card move clockwise

Electric Erection

Electric gyros can achieve higher RPM and are typically more rigid in space. Random drift rates are usually less than air driven equivalents.
Commutator switches on bearings energise torque motor.
Typical quoted limits 85° pitch and roll.

Remote Indicating Compass - Ch 15

Aka Gyro Mag Compass aka Gyro Compass aka Slaved Gyro Compass or Mag heading Ref System (MHRS)

Combines long term accuracy of the DIC with Short term accuracy of the gyro DG
Is a DGI with extra systems to keep it correctly aligned with Magnetic North (SLAVING)

Stator recreates the relative orientation of the Earth field around the drive shaft from the gyro unit to the display.

Slaving achieved by Flux Valve (Flux Detector aka Detector Element aka Magnetometer)
3 Horns (Mercedes Badge) - 120deg apart. Ferromagnetic material
Flux valve mounted on a Hooke’s joint to allow it to hang vertically, it does not rotate in the horizontal like a compass magnet but instead senses field direction.
Normally on wing tip or H stabliser - MIN DEVIATION
may incorporate cut-out switches to disable slaving during a manoeuvre (AoB >30deg)
‘SELSYN’S’ or ‘SELF-SYNCHRONISERS’ and are AC powered.

Components:

Detector Init (flux valve)
Heading ind (Annunciator -detects error)
Signal amp (amp'd to drive precession/torque motor on gyro) - 3deg/mi correction
Rapid synchronisation unit - if big error: 60 deg/min

AHRS

Consists of sensors 3 axes that provide attitude info for AC, including pitch ,roll, yaw
These ares sometimes referred to as MARG (Magnetic, Angular Rate and Gravity) sensors and consist of either solid state or (Micro Electro-Mechanical systems MEMS a combo of gyroscopes , accelerometers and magnetometers
EXAM: How many Gyros are in AHRS sysstems
ANSWER: 3
EXAM: In an AHRS system what does a the 3 axes magnetometer provide
A: The horizontal and vertical components of the earths magnetic field

Errors:

Generally RIC Errors are negligible
Turning
Acceleration
Deviation

Instruments Overview - Ch 1

MAAV - Pressure Instruments
CAT - Electro/mechanical
Analogue (Display) aka Comparative Display
Digital aka (Quantitive)
LRU (LINE REPLACEABLE UNIT) - ADC etc

Pressure Sensing - Ch 2

EXAM: Hectopascal is the most common flight instrumentation pressure
1mb = 1hPa
1bar = 1000mb = 1000hPa = 100,000Pa
Gauge pressure is the diff between pressure to be measured and atmospheric
ISA SL = 1013.25 hPa = 1013.25 mp = 29.92 inHg = 14.7psi

Measuring devices

Aneroid capsules (Lo Press) - sealed and ‘PARTIALLY EVACUATED'
Pressure capsule (Lo Press)
*Similar to aneroid capsule BUT has a pipe that allows static pressure to come in
Bellows = are multilayered pressure capsules*
Diaphragms (Med Press)
As pressure increases or decreases the diaphragm is moved
Bourdon tube (High Press)
An increased pressure makes the tube unravel…..just like a party blower

Pitot (Stagnation) pressure = Static + Dynamic

Measure Pitot Press

Pitot tube parallel to the airflow with the open end facing the airflow
MUST BE POSITIONED OUTSIDE THE BOUNDARY LAYER
Tube requires a drain to allow water to escape
Must have heating element to prevent icing
Large AC have 3 sets of pitots: captain , FO and standby

Measure Static Pressure

Static pressure sensed by a port that is PERPENDICULAR to the airflow
Site must be 'aerodynamically quiet’
Always comes in pairs, one each side of AC to eliminate effects of yaw
For unpressurised AC alternative (in emergency) static source is cabin

IMP

Instrument errors
Manoeuvre errors
Position errors

Pitot & Static:

Machmeter, ASI, ADC

Static only

VSI, Altimeter

Equations

Pressusre

Pitot (Stagnation) pressure = Static + Dynamic
Rate of Descent (fpm) = 5 * Groundspeed (kts)
For a 3 degree (~5% gradient) standard glideslope
Density Alt = Pressure Alt. + (ISA Dev. * 120ft)

Vertical Speed Indicator - Ch 4

Static source only
Measure rate of change of static pressure (fpm)
Logarithmic scale
Pressure capsule only
Metering unit produces a time delay of about 4secs
Metering unit - 2 parts (capillary(laminar) and orifice/knife edge(turbulent))

Climb (Press⬇️)

Low static press (Capsule) vs High press (case) = Capsule contract

Descend (Press ⬆️)

High static press (Capsule) vs Low press (case) = Capsule Expand

Errors

IMP
Lag - 4secs

IVSI only extra errors:

Turning error - would indicate that you are climbing
Oversensitivity in turbulence

Blocks and Leaks

Static Leak outside press. hull = slight inaccurate
Static Leak in press. hull = read VS for cabin press.
Static Blocked = Zero i.e. no rate of change

IVSI - counter LAG error

Dashpot/Accelerometer
Climb: Dashpot pushed down, increases volume of dashpot cylinder, decreases pressure in capsule = capsule contracts = climb indicated.

Additional Errors

Turning error - would indicate that you are climbing
Oversensitivity in turbulence

Rate of Descent (fpm) = 5 * Groundspeed (kts)
For a 3 degree (~5% gradient) standard glideslope

Pressure Altimeter - Ch 3

Static source only
Senses the static pressure at it’s location and compares its to a reference pressure
Sealed aneroid capsule only
Leaf spring helps control the rate of expansion and contraction
Climbing = When altitude increases, static pressure fed to case decreases. Aneroid capsule expands and altimeter increases

Altimeter Types

Simple
1 aneroid capsule
Sensitive
2+ aneroid capsules, jewelled bearings and VIBRATOR
Servo
EI Bar - capsule chg moves I, inducing current. Servo starts to move E back into position but also moves ind.

Adv = ⬆️accuracy, ⬇️friction, digital

Errors
Rate of change of pressure is non linear BUT alti calibrated to ISA

IMP
Temp
Hot to Cold, don’t be bold - Altimeter will Over-read

High to Low, look out below - Altimeter will Over-read
Barometric
High to Low, Look out below - Altimeter will overhead
Lag
*Response is not instantaneous
Climb may under read. Descent may over read
Hysteresis = Metal of aneroid capsule has natural resistance to change, esp. if no movement for a while.

Blocks and Leaks

Static Leak outside press. hull = slight over read
Static Leak in press. hull = UNDER READ (cabin press is HIGHER)
Static Blocked = Frozen display

Altimetry

QNH, QFE, SPS
Transition Alt., Transition Layer

ASI - Air Speed Indicator - Ch 5
REMEMBER TROPOPAUSE 36090FT = -56.5C!!!

Static & Pitot
Measure dynamic pressure ( Pitot - Static = Dynamic )
Logarithmic scale
Pressure capsule only
ASI will only read accurately in ISA conditions

Climb (Press⬇️)

Low static press (Capsule) vs High press (case) = Capsule contract

Descend (Press ⬆️)

High static press (Capsule) vs Low press (case) = Capsule Expand

Errors

IMP
Lag - 4secs

IVSI only extra errors:

Turning error - would indicate that you are climbing
Oversensitivity in turbulence

Blocks and Leaks

Static Leak outside press. hull = slight inaccurate
Static Leak in press. hull = read VS for cabin press.
Static Blocked = Zero i.e. no rate of change

IVSI - counter LAG error

Dashpot/Accelerometer
Climb: Dashpot pushed down, increases volume of dashpot cylinder, decreases pressure in capsule = capsule contracts = climb indicated.

Additional Errors

Turning error - would indicate that you are climbing
Oversensitivity in turbulence

Rate of Descent (fpm) = 5 * Groundspeed (kts)
For a 3 degree (~5% gradient) standard glideslope

Temp

TAT = SAT + Ram-Rise
But TAT doesn't account for TAT probe isn't 100% efficient for that we use RAT i.e. TAT corrected for inefficient probe
RAT (Ram Air Temp) = SAT + MRR
Where: MRR = RR x KR where KR = recovery factor

SAT = RAT (in Kelvin) / (1 + (0.2Kr *M^2) )

Convert Celsius to Kelvin on entry and exit to the formula

Airspeed

LSS (kts) = 38.95⎷K
M = TAS / LSS
M = ⎷(dyn press / static press)

Machmeter - Ch 6
REMEMBER TROPOPAUSE 36090FT = -56.5C!!!

Static & Pitot
Measure ratio of dyn. to static
Air speed capsule (pres. Cap) + Altitude capsule (Aneroid cap)
Ratio arm then Ranging arm
IF either capsule expands the indicated Mach ⬆️
IF either capsule contracts the indicated Mach ⬇️

Errors

IMP
BUT NO temp, density or compressibility errors cos affects both capsules so ratio is unchanged

Blocks and Leaks

PUDSOD

Display

MASI - Mach Airspeed Indicator aka Machmeter
NOTE and ASI without MACH is an ASI NOT a MASI!!!

Air Data Computer (ADC) - Ch 9

6 Inputs

Static pressure
Pitot pressure
RAT (measured temp)
AoA
Weight on the wheels
Configuration (flaps, undercarriage )

10 outputs

IAS and TAS
OAT and TAT
Mach Number
AoA
Altitude
Vertical Speed
Vmo/Mmo

Compass

Compass Error = Latitude + ( Angle of Bank / 2 )

Pneumatic erection

COARSE erection: Erection jet blowing from outer gimbal onto rotor rim spins gyro and produces a force component that erects the gyro relative to the aircraft.
FINE erection: Air exhaust from gyro unit blows over wedge on the outer gimbal. Balanced force if gyro erect relative to aircraft, unbalanced otherwise.

Slow rate of precession (3deg/min) to compensate for:
- Random Wander (mechanical imperfections)
- ER
- TW (caging - done by pilot)

Turning

Rate 1 turn = 3deg/min = 360 per 2min
Rate 1 turn is 2mins
Circumference = TAS (kt) / 60 * 2mins = 2piR

LEARN COCKPIT X-CHECK Qs!!!
Learn 'Hi to low', 'Hot to cold', etc..!!!

REMEMBER TROPOPAUSE 36090FT = -56.5C!!! when doing temp calcs or CRP5!!