What’s wrong with this load chart?

While studying for the performance exam, I decided to get out an owner’s manual I have for a Cessna 172 (1975 M model) and thought I would practice a few weight and balance questions. Using the sample loading problem it was all pretty simple, in fact using their sample weight of 1366lbs and 53.8" moment it was easy to balance the aircraft with any number of combinations of pilot and passengers (I will try and attach the graph)

However I then decided to play with some real numbers, by coincidence our aero club has just acquired an M model 1974 C-172. So from the POH I obtained the licenced empty weight of 650.73 kg and 1001.6 moment. I converted these figures to 1434Lbs and 39.4" moment.
So here is the problem, using these numbers, I don't seem to be able to balance the aircraft at all, the CofG seems too far forward in all cases, even using a standard pilot and co pilot both at 77kg with full fuel.

Try a couple of scenarios for yourself? It doesn’t seem to matter how many passengers or how much fuel, it’s always out of balance, unless you put a pilot and a load of ballast in the back.

Can anyone see what I’m doing wrong?

Hi Rob,
I don't think you are doing anything wrong. I think there something wrong with the data figures.
When was the last time the aircraft was weighed? Has there been any significant changes like the addition of an autopilot? Something weighing nearly 31kg has been added to it to give it a moment figure of 39.4"lbs.

There should a stamp on the P-charts and W+B data certifying the last time it was checked with any ammendments such as the empty weight moment.

You could ask the LAME who maintains it! They might know.

Hope this helps.

Mister W.

Thanks,

I will take your advice and speak to the LAME. This particular aircraft has not been put on line yet, it is currently undergoing SIDS inspection. I had been using the P charts from a C-172 owners manual I had bought on line (just for practice) it of course is all in imperial measurements, moment in pound-inches etc.

When I looked in the POH for the aircraft it did not contain any p charts just the basic empty weight in Kg and a moment of 1001.6mm, which if I'm not wrong converts to 39.4" which is a long way from the example in my owners manual, perhaps the charts for this aircraft are very different from the one in my owners manual, although you would think all 1974/1975 M model C-172's would have the same charts, just different weights and moment arms, depending on specs.

Just as an interesting side point, I tried the same W & B problem using Ozrunways on my Ipad, they have a generic C-172 profile which uses 39.3" moment arm, once again a long way from the 53.8" in the sample problem from the owners manual.

Thanks again for your help, I will see what the LAME has to say.

Hi Rob,
Disregard the bit about P-charts. Long night at work followed by renovating the house. usually, it's only the W+B stuff that gets re-weighed and certified periodically.
Let us know what you find out.

Mister W.

I figured it out. I was using 39.3" this is the arm not the moment. I had to multiply by the weight of 1434lbs to give a moment of 56.56. Now it all works. ( yes I feel stupid )