Envelope vs Limits

Windchill

Well-Known Member
SEL Private Pilot for a month now, planning a trip for me and my fiance with a friend from work and his wife to go to Atlantic City.

Did the calculations but here is the problem, we make weight, I have us starting with 3/4 tank and going to make a pit stop on the way to and from,

however,

when it comes to Center of Gravity (1983 Cessna 172 P), we would fall WITHIN the boundaries of the Center of Gravity Moment Envelope but OUTSIDE of the Center of Gravity limits.

What's would cause this and what's the difference 'tween:

Center of Gravity Moment Envelope vs Center of Gravity Limits?

Appreciate any help with this!

These are just two different ways of expressing the allowable loading range, and there shouldn't be any difference between the two. You probably should just recheck your calculations - either calculation should result in a point which is the same in terms of percentage of allowable limits on both charts.

I noticed in examining the loading charts for a 1979 172N (very similar to your P) that there is a little bit of difference in the extreme aft limits of the envelope. For example, at the gross weight of 2300 lbs., maximum moment is about 109,000 lb-in. However, in checking the center of gravity range, the aft limit appears to be about 47.3 inches at 2300 lbs., which would correspond to 108,790 lb-in. That difference is basically within a reasonable rouding error and the precision to which the charts can be read. Are you really close to the gross weight or envelope limits?

To the extent there is truly a discrepancy between the two, the manual seems to prefer or indicate the center of gravity moment envelope as the method to use to make a go-no-go decision.

Thanks for the reply. I'm still throwing caution to the wind and holding on until I can get a hold of my instructor from PPL training and ask him what he thinks and says.

Better to be prudent than hasteful!

[ QUOTE ]
Thanks for the reply. I'm still throwing caution to the wind and holding on until I can get a hold of my instructor from PPL training and ask him what he thinks and says.

Better to be prudent than hasteful!

[/ QUOTE ]

That's a great idea. In the meantime I'd recalculate it anyways and check for errors. There's no reason why Cessna would make two different loading requirements depending on the chart you use, they're just two different ways of saying the same thing.

Couple of quick things:
-When is the trip?

-If you're areadly really close to the CG limits, do you think it'd be better to just move some stuff so you're not so close? Remember the CG will move as fuel burns so taking off near or at limits may mean landing outside of limits which could make the flare difficult.

The trip isn't scheduled for anytime soon, nor planned for any point in time, just a thought a few of us had.

I did take into account the difference when it came time to land, and everything checks out fine. I went over the numbers a couple of times and got the same results, even moved seating locations around and dropped fuel load to 3/4 and still didn't make it . . . will recheck though.

You are within gross weight, right? If you are not over it shouldn't be all that hard to keep it in the CG limits for a 172. Are you showing too far forward or too far aft?

too far aft

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too far aft

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Ahhh, now you know what its like to plan a trip in a Bonanza. That thing is always aft...and if its not when you take off, theres a good bet that it will be before you land.

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