W&B and minor alterations

[Richard]

FAR 21.93 ... A "minor change" is one that has no appreciable effect on the weight, balance, structural strength, reliability, operational characteristics, or other characteristics affecting the airworthiness of the product. All other changes are "major changes" (except as provided in paragraph (b) of this section).

Say a 30 year old C-172 has acquired multiple minor alterations over the years but none required any thing other than a single logbook entry, what can be expected of their culmative effect on W&B? At what time, if any, does the aircraft become not in compliance with mfg's design?

 

[Chache]

FAR 21.93 ... A "minor change" is one that has no appreciable effect on the weight, balance, structural strength, reliability, operational characteristics, or other characteristics affecting the airworthiness of the product. All other changes are "major changes" (except as provided in paragraph (b) of this section).

Say a 30 year old C-172 has acquired multiple minor alterations over the years but none required any thing other than a single logbook entry, what can be expected of their culmative effect on W&B? At what time, if any, does the aircraft become not in compliance with mfg's design?

It would seem, their culmative effect on W&B would be to increase the weight, and their placement, to change the balance.

And, that the aircraft would not be in compliance with mfg's design, when the cumulative effect of all the minor alterations on the aircraft, affect the weight, balance, structural strength, reliability, operational characteristics, or other characteristics affecting the airworthiness of the product.

Kind of like the straw, that broke the camel's back.
When did the load of straw, actually break the camel's back??
With the addition........... of that last piece, which exceeded the camel's capacity.

Anyone disagree?

[edit]
But I'd think there would have to be ahellovan awful lot of them to make any difference at all...

 

[Henning]

It would seem, their culmative effect on W&B would be to increase the weight, and their placement, to change the balance.

But I'd think there would have to be an awful lot of them to make any difference at all

Anyone disagree?

Well, I wouldn't assume the weight would go up, as lot's of upgrades, especially to the panel, are much lighter today than in 4 decades old stuff, and various other mods such as the 210 gear door elimination mod save you weight. It could go either way, mostly depends on how advanced the avionics for the vintage were. A Luscombe has probably gained weight from it's manufacture date. A B model 310 that have the full avionics suite including radar and radar altimeter, big Sperry gyros and autopilot, full airline style executive instrumet panel... that has new generation avionics to the same caliber and ability (plus ability unknown at that time), it's probably lost weight.

I would think that a person could keep it reasonably correct through many years by exercising due care when doing the mods. I figure if major mods get a reweigh, and I perform a weighing when I buy it (even if it's just using a bathroom scale) I would always feel confident that I am operating on correct information.

 

[Stache]

This is my opinion so I would like to start with a passage from AC 43.13-1b Chapter 10 Weight and Balance paragraph 10-2(c)

Negligible Weight Change is any change of one pound or less for aircraft whose weight empty is less than 5,000 pounds; two pounds or less for aircraft whose weight empty is more than 5,000 and 50,000 pounds; and five pounds or less for aircraft whose weight empty is more than 50,000 pounds. Negligible c. g. change is any change of less than 0.05% MAC for fixed wing aircraft, 0.2 percent of the maximum allowable c. g. range for rotary wing aircraft.


So if the aircraft is 5,000 lbs or less you are allowed up to one pound and not have to record it. But the questions is how many one pounds are you allowed before your C.G. is out? I agree with the cumulative effect
because aircraft gets heaver as they age.


Because of the cumulative effect
is why the part 135 and 121 aircraft are required to be weighted every 36-months. I would think if an aircraft has not been weighed in over 10-years and has had many alterations it should be weighed to find out what the base line is.

Stache

 

[poadeleted20]

FWIW, we did an unofficial weigh-in for Grummans a couple of years ago -- just total weight on certified scales, but no leveling to get accurate cg. The airplanes all had FAA-legal W&B data including various computed changes and "negligibles" over the years. On average, they weighed about 25 lb more than the "official" document said. 1% over is probably not, by itself, enough to get hurt, but it's interesting, and if you start nudging the edges with additional overages you really could overdo it. For that reason, I think Stache has a real good point in recommending a weighing every 10 years whether you "need" it or not.

 

[wsuffa]

Recommended? OK. Required? I'd be opposed.

For one, the manufacturer of my aircraft changed the weight procedure sometime after my plane was made. Assuming that the original procedure was used to determine certification limits, then that procedure should be more in line with certifications. Problem is that the new procedure generally indicates that the plane is heavier than the old procedure.

Old procedure: Drain fuel, weigh empty, calculate weights assuming the specified unusable/undrainable fuel.

New procedure: Fill tanks full, weigh full, subtract usable fuel capacity in pounds to obtain empty weight.

The fuel tanks on the plane hold somewhere between 3 and 4 gallons apiece more than the book usable fuel, meaning an automatic loss of 36 - 48 pounds of useful load.

The highly conservative approach is to use the mew method. But what was really certified was the initial method.