Optimimum Cruising Altitude - C172

[MFALK]

Was looking at the cruise perf. specs. for the C172. Am I correct in saying that for short trips (100-200Nm), there is really no need to cruise higher than 2000-3000ft as there is little or no gain in fuel economy (unless of course you get a better tailwind at the higher altitudes)?

[Luftwaffle]

I have a performance table from a C172A. Calculated on a bent E6B, for a 150 nm trip.

Unfortunately PPRuNe strips extra spaces, so this table doesn't line up in two columns, like I typed it.

cruising altitude 3000' 8000'
time in climb (min) 5 17
distance in climb (nm) 6 22
fuel in climb (US gal) 1.0 3.1
TAS in cruise (kt) 109 106
time in cruise (min) 79 72
cruise fuel burn (gph) 7.7 6.7
fuel in cruise (US gal) 10.1 8.0
total time (min) 84 89
total fuel (US gal) 11.1 11.1

Conditions:
power full to climb, 2400 rpm in the cruise
no wind
standard temperature
start at sea level
leaning per POH
no allowance for reduced power in descent

Conclusion: a 172 is not a jet. Pick a legal altitude that gets you over everything in your way.



[This message has been edited by Luftwaffle (edited 24 September 1999).]

[redsnail]

I always thought that TAS increses with altitude.
In Oz in Summer up north, if you flew at 2-3,000' you would guarantee a very rough ride.

------------------
reddo
Life is good 'till the next stuff up.

[Vx]

In the pilots operating handbook you should find a TAS / altitude table for the aircraft which should indicate a peak TAS at cruise power around 5000'-7000'. (You're the student - you work out why !!) What you want really is best groundspeed so drag out the met and work out g/s for all feasible altitudes.