Didactic explanation of how to use the most important feature of the computer side of a Jeppessen CR3

1. CR-3 CALCULATOR SIDE
1. TIME - DISTANCE – SPEED
a.
GIVEN:
1. Place the GS (20 standing for 200)
• GS = 200mph opposite to the TIME INDEX
• DISTANCE = 300sm
2. Opposite to the DISTANCE (30 standing
FIND: for 300) on the outside (white) scale
TIME ENROUTE on the inner grey scale
• TIME ENROUTE
ANSWER: 90 min or 1:30h
b.
GIVEN:
1. Place DISTANCE opposite TIME. The
• DISTANCE = 210sm later on the inner scale
• TIME ENROUTE = 50min 2. Opposite to TIME INDEX GS
FIND: ANSWER: 252mph
• GS
NOTE: Any figure on the outer scales (white and grey) of the computer can stand for
any number containing the given digits: i.e.: 40 can stand for 0.4, 4, 40, 400, ...
2. FUEL CONSUMPTION
Is worked out in the same manner as 1
• Place FUEL CONSUMPTION opposite to TIME INDEX
• Place FUEL CONSUMED (Gallons) on outer scale
• Place TIME OF OPERATION OF THE ENGINE on inner scale
GIVEN: 1. Place on the outer scale the amount of
FUEL CONSUMED
• FUEL CONSUMED = 105 USGall
2. Opposite to it, on the inner scale, place
• TIME = 1h 30min
the TIME
FIND: 3. Read FUEL CONSUMPTION on the outer
scale opposite to TIME INDEX
• FUEL CONSUMPTION
ANSWER: 70 USGall/h

2. 3. CONVERSIONS
Place the arrow of the first unit of measure on one scale opposite the arrow of the other
unit on the other scale. Read corresponding values opposite each other on the two scales
SPECIAL CASES:
• ºC - ºF Special conversion scale
• Weight of fuel and oil
• Minutes to seconds Time index opposite to the blackgrounded 10 and read
minutes on the outer white scale and seconds on the inner grey scale
4. ALTITUDE
• INDICATED ALTITUDE = Reading on the altimeter set with QNH
• CALIBRATED ALTITUDE = Indicated altitude corrected for instrument, position and
installation errors.
• TRUE ALTITUDE = Calibrated altitude corrected for non-standard atmospheric
conditions
• PRESSURE ALTITUDE = Indicated altitude when set with QNE
• DENSITY ALTITUDE = PA corrected for non-standard temperature
a. DENSITY ALTITUDE (Near the centre of the computer)
GIVEN. 1. Place PA in front of TAT
2. Read DA 5000’
• PA = 3000ft
• TAT = (25ºC)
FIND:
• DA
b. TRUE ALTITUDE (Use the calibrated altitude and TAT)
Greater accuracy can be obtained if you also know the altitude of the ground
station giving you the altimeter setting.
GIVEN: 1. Place PA opposite to TAT
2. Substract to CA the GSA in
• PA = 10000ft order to get the CA above
• CA = 9000ft ground level
• TAT = -20ºC 3. Opposite to CA above ground
• Ground station altitude = 5000ft
on the inner scale is TA
FIND: above ground on the outer
scale
• TRUE ALTITUDE (TA)

3. 4. Add station altitude to TA above ground to get TRUE ALTITUDE ABOVE SEA
LEVEL.
5. If station altitude is at SL or unknown, find the TA on the outer scale
opposite to CA on the inner scale
5. TRUE AIRSPEED
On fast flying aircraft the air is compressed in front of the aircraft and is heated by
compression (OAT bulb feels a higher temperature than what really exist in the
surrounding non-compressed air). Also the rush of air over the OAT bulb creates a friction
causing further heating and still higher (false) reading.
This increase in temperature TEMPERATURE RISE
NOTE: Even though both knots and mph can be used, a more accurate TAS answer will be
given using knots
Following parameters are necessary
1. CAS (or IAS if unable)
2. PA (or altitude if unable)
3. Indicated OAT (ºC)
TEMPERATURE RECOVERY COEFINCIENT (Ct): Varies with installation and design of the probe for
every aircraft (0.6 – 1.0). Can be considered constant even though it varies slightly with speed
and altitude CR-3 only consider Ct 0.8 or 1.0
• SOLID LINE (Ct=1.0) Standard stratosphere temperature (-55ºC)
• DASHED LINE (Ct=1.0) Standard SL temperature (+15ºC)
Between them interpolate
NOTE: If not otherwise stated will assume Ct = 1.0
a) DIRECT METHOD
GIVEN: 1. Place CAS opposite to PA
• CAS = 400kts 2. Ct 1.0 line crosses intersection between SPIRAL
• PA = 15000ft LINE and INDICATED TEMP wiggly line
• IAT = 30ºC 3. Read TAS underneath
FIND: ANSWER: 500kts
• TAS

4. MACH NUMBER: Read at the pointer indication directly beneath TAS
Represents different TAS at different Temperatures
b) TAS FROM M NUMBER AND TEMPERATURE
GIVEN: 1. Place M # in front of M # ARROW underneath
TAS
• M = 0.55 2. Set Ct line over intersection of SPIRAL with
• TAT = -15ºC TEMP (IAT)
FIND: 3. Read TAS in knots under cursor line
• TAS ANSWER: 673kts
NOTE: If OAT is not available is possible to use reported or estimate air
temperature (the result will be as accurate as the estimation)
c) DOUBLE-ENDED MACH INDEX ARROW
Relates ISA altitude with ISA temperature (ESTIMATED TAT)
GIVEN: 1. Set MACH INDEX ARROW in front of PA in ISA
scale
• PA = 28000ft
2. Read on the other side of the arrow ISA temp
FIND: for that PA
• ESTIMATED FREE AIR ANSWER: -40ºC
TEMPERATURE
d) TAS FROM TAT
1. If the aircraft is equipped with a M # indicator and you know the TAT
GIVEN:
1. Place M # ARROW opposite to TAT
• M # = 0.55 2. Opposite to M # on inner scale read TAS on outer
• TAT = -15ºC scale.
FIND:
ANSWER: 344kts
• TAS

5. 2. If the aircraft is equipped with conventional airspeed indicator:
1. Use the direct relation method matching CAS with PA
GIVEN:
and finding the M # on the M INDEX section
• PA = 14500ft underneath the TAS
• CAS = 280kts 2. Follow the steps above ANSWER: 343kts/0.55
• TAT = -15ºC
FIND:
• M # / TAS
TEMPERATURE RISE: Probes nowadays use recovery coefficient of 1.0. But, if temperature rise is
desired for a temperature probe with a Ct = 0.8, the Ct = 0.8 must be used, and the value for the
“TEMPERATURE RISE Cº(CT 1.0)” scale multiplied by 0.8
1. Place CAS opposite to PA
GIVEN:
2. Move green cursor until hair line passes through
• CAS = 276kts intersection of IAT line and SPIRAL line.
• PA = 10000ft 3. Read Trise on the section bellow MACH NUMBER
• IAT = 0ºC 4. Must take into account: IAT = TAT +Trise
• CT = 1.0
ANSWER: Trise = 13ºC / TAT = -13ºC
FIND:
• TAS /Trise
e) “OLD METHOD” TO FIND OUT TAS
This method doesn’t correct for temperature rise nor compressibility and ,
though, is not suited for high speed calculus (<200kts)
GIVEN: 1. Place PA opposite to TAT
2. Opposite to CAS on the inside grey scale, read
• CAS = 166kts
TAS .
• PA = 5000ft
• TAT = 10ºC ANSWER: 180kts
FIND:
• TAS