1975 Cessna 172M

[Jon Wilder]

Hi all. So I've begun my pilot training in a 1975 Cessna 172M. In the checklist published in the POH, I see that there's Normal Takeoff (self-explanatory), then there's Maximum Performance Takeoff. The procedures in the Max Performance Takeoff very closely resemble the Short Field Takeoff procedures in the 172S. However, the Short Field Takeoff in the 172S says "Wing Flaps - 0* - 10* (10* preferred), whereas the Maximum Performance Takeoff procedures in the 172M state Wing Flaps - UP.

As a matter of fact, I cannot find any place in the 172M POH where it even says you can take off with any flap setting other than UP.

Furthermore, the newer 172S model also gives a higher Vfe for 10* flaps whereas the 172M uses 100 MPH Vfe for all flap positions other than 0.

And yes I am aware that our 172M uses MPH (this variant was a transition variant, where later 172M's use KTS instead of MPH) while the S uses KTS.

What, if any, are the differences in airframe and flap design between the 172M and the 172S? Is there any harm in using the V speeds and procedures from a 172S POH for a 172M? Or are they different enough to where that wouldn't be adviseable? Our 172M has a Garmin G3X PFD/MFD setup, so the airspeed indicator can be programmed to use either MPH or KTS, but we currently have it programmed for MPH as the POH speeds are published in MPH.

Thanks in advance.

 

[texasclouds]

Use the V speeds and flap settings per the manual of your aircraft. Don’t mix and match!

 

[Jim Carpenter]

I have a 1974 172M POH at home, pretty sure the 1975 version is the same. You have to look in Ch. 2 (Page 2-12 in my book) at the verbal description of takeoff procedures/wing flap settings. It says (paraphrasing): "normal and obstacle t/o done with flaps up. 10 deg. flaps shortens ground run, but advantage lost in climb, so 10 deg. is reserved for soft-field." And, further on, "Flap settings greater than 10 deg. not recommended at any time for t/o."
Also, in the Ch. 4 airspeed limitations section, as you say, it gives a Vfe of 100 mph, with no other speed for flaps 10. This could simply be due to the fact that they never certified or tested flaps 10 at higher speeds like the newer 172s, but, there it is in black and white as a limitation. It could very well be fine to go flaps 10 at a higher speed, but I wouldn't make the assumption.
The older POHs are quite a bit less informative than the new versions, often you have to hunt for some of the V speeds in the text, or the performance charts are less than thorough, etc.

 

[brcase]

Cessna appears to have changed the short field TO procedure about mid production of the 172N Model. Early N model manuals mirror what Jim posted about the 172M but the speed are converted to knots but still about the same. Late production 172N model manual show the short field take-off using 10 degrees of Flaps and a slower Vx speed for take off.

I don’t have an official source but my pet theory is that they changed the procedure because it made only a small difference in the performance, or perhaps more testing showed no difference I haven’t compared the N model manuals to see if the take off distance performance numbers changed. But using 10 Degrees of flaps make it more similar to the C182 procedures, so by training to use 10 degrees of flaps in the 172 it does make the transition to the 182 slightly easier.

IIRC I think the 172N model was the 1st model to have the increased flap speed for Vfe for up to 10 degrees of flaps. But as Jim said above it is clear in the manual that Vfe is 100MPH and I am not aware of any document that allows or even hints that any higher speed is acceptable or allowed. I don’t know of any structural changes, but the other changes to flap procedures on the N model might hint that there was some change.


I looked at the TCDS for the 172N, only hint to the change there is a couple for the V speeds (Not Vfe) changed for the 1980 mode. But as I recall it was the 1980 model that showed a different Short Field procedure from earlier 172N Models.

Brian
CFIIG/ASEL

 

[Jon Wilder]

Thanks for all the input guys! @Jim Carpenter after reading your post I did find that section in Section 2 of the 1975 172M POH. Page 2-14 in the 1975 POH.

 

[Zebranaut]

I know I'm a bit late to the party, but I thought this explanation from our 1976 172M's POH (4-14) might be useful. 10° is allowed but only recommended for soft fields. On this model, flaps shorten the ground run a little, but hurt the 50' obstacle clearance.