Mach number to IAS..HELP
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Mach number to IAS..HELP
An aeroplane cruising at FL 350 in ISA conditions commences its descent and holds Mach 0.76 to an altitude where its speed is 300KIAS at which point the pilot selects airspeed hold for the remainder of the descent. Using a plot of True Airspeed at altitudes between top of descent and 20,000 ft determine the altitude where Mach Number and Indicated Airspeed correspond.
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MN = TAS / LSS
At 35000' ISA temp = -55 degrees C
add 273 to get Kelvin = 218 K
LSS = 38.94 sqr root (Temp K)
LSS = 575
MN = TAS / LSS
0.76 = TAS / 575
TAS = 0.76 x 575
TAS = 437
repeat this at 5000' lower intervals for example to get a plot of TAS with altitude down to 20000'
Use the CRP5 to calculate corrected TAS with altitude from 20000' to 35000' using an IAS of 300 KT
The lines will cross over
At 35000' ISA temp = -55 degrees C
add 273 to get Kelvin = 218 K
LSS = 38.94 sqr root (Temp K)
LSS = 575
MN = TAS / LSS
0.76 = TAS / 575
TAS = 0.76 x 575
TAS = 437
repeat this at 5000' lower intervals for example to get a plot of TAS with altitude down to 20000'
Use the CRP5 to calculate corrected TAS with altitude from 20000' to 35000' using an IAS of 300 KT
The lines will cross over
If holding .76 while descending. the TAS will not be constant due to the increasing TAS associated with the presumably increasing temperature as you descend. Another way to approach the problem might be to calculate the altitude at which 300 KIAS results a TAS which at that temperature is equal to .76 mach. I've sometimes seen or heard this is referred to as mach/IAS crossover altitude. I also seem to remember seeing a chart in an operational planning manual somewhere too. No matter.
Whenever I've gotten curious about crossover altitude I just estimate it with mental math. Close enough.
Using 2% per thousand feet as the approximate increase of TAS/IAS with altitude increase, track 300 KIAS to 25,000' to get about 450 KTAS. Now just assume the speed of sound to be about 600 kts at that altitude. 76% of 600 is just more than 450. Crossover will be near FL250.
Wanna check to see if this is the actual number? First get get out the calculator. 38.94 (or 39) x sq rt temperature in deg K at 25,000. calculate TAS of 300 KIAS at 25,000'. You could input the formulas into excel and make chart.
Or use your favorite search engine and the search terms "atmospheric properties calculator" to find a pretty handy online calculation tool.
Have fun!
westhawk
Whenever I've gotten curious about crossover altitude I just estimate it with mental math. Close enough.
Using 2% per thousand feet as the approximate increase of TAS/IAS with altitude increase, track 300 KIAS to 25,000' to get about 450 KTAS. Now just assume the speed of sound to be about 600 kts at that altitude. 76% of 600 is just more than 450. Crossover will be near FL250.
Wanna check to see if this is the actual number? First get get out the calculator. 38.94 (or 39) x sq rt temperature in deg K at 25,000. calculate TAS of 300 KIAS at 25,000'. You could input the formulas into excel and make chart.
Or use your favorite search engine and the search terms "atmospheric properties calculator" to find a pretty handy online calculation tool.
Have fun!
westhawk
Are you permitted to use an ISA chart for this question?
If so then it will include columns for the TAS at LSS, and the relative density at various altitudes.
Just go down the LSS column multiplying the TAS by .76 to get the TAS for Mach0.76 at each altitude.
IAS = TAS x square root of relative desnity. So just go down the relative density column multpilying 300 KIAS by the square root of relative density to get the TAS at 300 KIAS.
Now sketch two lines to represent the values calulated above. The two lines will cross at the required altitude.
From a quick check using an ISA chart provided for JAR exams I make it 26000 feet from
If so then it will include columns for the TAS at LSS, and the relative density at various altitudes.
Just go down the LSS column multiplying the TAS by .76 to get the TAS for Mach0.76 at each altitude.
IAS = TAS x square root of relative desnity. So just go down the relative density column multpilying 300 KIAS by the square root of relative density to get the TAS at 300 KIAS.
Now sketch two lines to represent the values calulated above. The two lines will cross at the required altitude.
From a quick check using an ISA chart provided for JAR exams I make it 26000 feet from
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Thanks alot everyone for your replies..This has been great help, i was stuck for a couple hours pulling my hair out until i finally thought someone might be able to help me on here and use have. Thank You 
