Weight and Balance on new plane

jd21476

jd21476

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jd21476
I am trying to enter the weight and balance on my new plane in to Foreflight and I want to confirm I am reading this diagram correctly.

1. The + just below the engine is the DATUM, correct?
2. The front seats ARM is 78" - 29.7" = 48.3"?
3. The fuel and rear seats are 78" from the DATUM?

If this is all correct, what does the 79" represent?
 

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jd21476 said:
The + just below the engine is the DATUM, correct?
Click to expand...
Not quite. The datum is the line with the word "DATUM" next to it or 79" forward of the leading edge of the wing. The "+" you see along with the other "+" mark the center of the wheels..... A quick look to the TCDS will more define the DATUM, seat locations, baggage area, etc. for you.
 
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The 79" shows that the datum is 79 inches forward of the wing leading edge.
The "+" is the axle center for the landing gear wheels.
The 29.7" is the distance between the wing leading edge and the center of the main gear wheel axles.
The 78" is the distance between the axle centers of the nose wheel axle and the main gear wheel axles.
 
That seems awefully forward for the DATUM, I thought the DATUM was 'usually' the firewall.
 
jd21476 said:
I thought the DATUM was 'usually' the firewall.
Click to expand...
On older aircraft it usually was the firewall or in some cases the spar. Most "newer" aircraft have the datum at the nose or projected at a point in front of the nose which is a much better place. Trust me. This helps reduce math errors during W&B calculations by getting rid of double negatives, etc.
 
Ok, sounds good to me.

So not to sound completely stupid but how would I then find the distance from the DATUM to the fuel and the rear seats?

The fuel is in the wings which is essentially over my rear landing gear so can I just use that measurement and the back seats as well?
 
jd21476 said:
how would I then find the distance from the DATUM to the fuel and the rear seats?
Click to expand...
The answer lies in Post #2. Hint: TC_ _.;) Plus I'm sure your POH/AFM has that info in the Weight & Balnce Section somewhere?
 
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So I went in to the TDCS and pulled all the data I need, thanks for all the help.
 
The datum is whatever imaginary point the manufacturer decided to use. It could be the firewall, the spinner tip, or some imaginary point in front of the airplane. Using the firewall is problematic, because there can be changes ahead of the datum requiring negative moments. The point is that everything gets measured from the same point.

The book should provide arms for front and rear seats, baggage compartment, and fuel tanks.

FWIW, in the PA24-250 (and I believe the 180 as well)

Front Seats - 84.8 inches
Rear Seats - 118.5 inches
Baggage - 142.0 inches
Main tanks - 90.0 inches
Intergral aux tanks - 95.0 inches
 
One thing you will find with the Comanche, its darn near impossible to get it out of CG. You can be overweight just like any other light single, but you'd have to try to get out of CG.
 
midwestpa24 said:
The datum is whatever imaginary point the manufacturer decided to use. It could be the firewall, the spinner tip, or some imaginary point in front of the airplane. Using the firewall is problematic, because there can be changes ahead of the datum requiring negative moments. The point is that everything gets measured from the same point.

The book should provide arms for front and rear seats, baggage compartment, and fuel tanks.

FWIW, in the PA24-250 (and I believe the 180 as well)

Front Seats - 84.8 inches
Rear Seats - 118.5 inches
Baggage - 142.0 inches
Main tanks - 90.0 inches
Intergral aux tanks - 95.0 inches
Click to expand...

The TDCS for the 180 showed:
Front Seats - 85 inches
Rear Seats - 118.5 inches
Baggage - 142.0 inches
Main tanks - 90.0 inches
 
jd21476 said:
The TDCS for the 180 showed:
Front Seats - 85 inches
Rear Seats - 118.5 inches
Baggage - 142.0 inches
Main tanks - 90.0 inches
Click to expand...

Hmmm, does the 180 have 2 tenths of an inch more legroom in the front seat? :p
 
jd21476 said:
The TDCS for the 180 showed:
Front Seats - 85 inches
Rear Seats - 118.5 inches
Baggage - 142.0 inches
Main tanks - 90.0 inches
Click to expand...
Remember, the TCDS is the official definition for soooooo much of the physical definition and limitations of the aircraft
 
murphey said:
Remember, the TCDS is the official definition for soooooo much of the physical definition and limitations of the aircraft
Click to expand...
Sort of. I have found discrepancies between the TCDS and POH for some airplanes. The 185, for instance, shows somewhat different arms for the fuel tanks, and some airplane that have long-range tanks don't show that in the TCDS. I would trust the POH over the TCDS, as that's what the enforcement guys would expect of a pilot. Mechanics need to use both.
 
Dan Thomas said:
Sort of....I would trust the POH over the TCDS,
Click to expand...
FWIW: And only at you're peril with those enforcement guys. The TCDS is part of the TC/Type Design and is the sole document used when it comes to issuing AWCs, determining airworthiness, etc. Also not all configurations need to be in the TCDS--that's actually up to the OEM. While I understand you're point and while there is conflicting guidance on which document to use (TCDS/POH/AFM/MM) in certain cases, it's been my experience to stick with the TCDS or the approved Limitations Section in the POH/AFM when making determinations.
 
Bell206 said:
FWIW: And only at you're peril with those enforcement guys. The TCDS is part of the TC/Type Design and is the sole document used when it comes to issuing AWCs, determining airworthiness, etc. Also not all configurations need to be in the TCDS--that's actually up to the OEM. While I understand you're point and while there is conflicting guidance on which document to use (TCDS/POH/AFM/MM) in certain cases, it's been my experience to stick with the TCDS or the approved Limitations Section in the POH/AFM when making determinations.
Click to expand...
I never encountered the TCDS until I became a mechanic. It didn't show up anywhere in my PPL or CPL or instructor training. This is Canada, though. Maybe the FAA demands some initiation in the TCDS for pilots?
 
Dan Thomas said:
Maybe the FAA demands some initiation in the TCDS for pilots?
Click to expand...
In a round-about way yes. But it falls more into the gray area of aircraft ownership that is not formally taught but implied within the FARs. I've always been a fan of requiring PPLs to learn what 91.403(a) really means. Unfortunately, the only time when most people, owners or mechanics, find out about the TCDS is when something happens and they're laid out on the table. For example, would you ensure an aircraft meets its "applicable type certificate data" if you were to sign off a FAA annual or required inspection?
 
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