Diet plan for skymasters?
 

I'm looking at a pre-73 Skymaster that's about 250lbs over the "factory" weight. It's got a 400 autopilot, old radios, Robertson STOL but nothing obvious. Is this common? Can I reasonably expect to put the plane on a diet (modern avionics, rewire, etc) and get 150-200lbs back?

It's a 337D. Factory published weight is 2655, this one is near 2900.

Absolutely not. If you completely strip out EVERY obsolete wire and old avionics you'll be lucky to net 20 pounds in recovered empty weight.

Jim

I think you are asking for a lot with just avionics upgrades. Have you seen any W&B history for this plane? If there is any history this might clue you in to something that added a lot of weight at some point. With avionics update I would guess you might gain 20 or 25 lbs, that would be a lot.

There are lots of mistakes made in W&B calculations too. Look closely at the sheets

Jim, it's not uncommon for people to ask if they should do an actual re-weigh or just go with a calculated W&B after work has been done.

The usual answer is to "prepare to lose some useful load." I've not seen a legacy GA aircraft yet that was as skinny as the factory weigh.

BTW, was your bird repainted? Paint is REALLY heavy if put on too thick. Also, I was very surprised how much my carpet and underpad weigh after 40 years of moisture, hydraulic fluid, etc.

I'm pretty sure Cessna, and most other GA manufacturers, never weighed planes on the line and instead based their numbers on a perfectly built theoretical plane. Add in years of rounding errors and absorption of fluids by fabric and insulation and it's not surprising that the theoretical and actual weights can differ.

I gained about 60lb doing a major upgrade on my 340. One single box that wasn't even hooked up (old inverter) was 11lb by itself.

I'll going to ask for the history on this one. 250 over factory weight seems fat - I'm guessing I might find a typo. After my upgrade we did a reweigh, and I found a typo right away in the signed W&B that they put in my logs.

I'm big on "the diet" as well. Over the years my plane had a few different radios, ADF's and whatnot installed in it.

Its pretty common for shops to run new wiring and just cap the old stuff, rather than spend the time to remove the old wiring. I've done half of my insti panel, gutted as much of the old stuff as possible. Removed old ADF systems, DME and other stuff that can be replaced with modern gear, and literally a box full of wiring.

My (calculated) change was about 21 lbs.

If you really wanted to save weight, the obvious solution would be to ditch the vacuum system, and go to a DC and/or Glass install. Then 3 bladed props. Then... then... then...




There is no problem that cannot be solved with the correct application of money.

Leighton

Radios and RSTOL
 

I have a turbo T337D with RSTOL.

I eventually replaced my factory original RT485 radios (I think that was the number), when one of the gears broke.

Each original 'radio' was actually just a head, which then connected each to its own radio in the avionics bay, each of those having its own power supply. I don't remember the number, but that pile of stuff weighed a lot when it came out.

With the Robertson STOL (which I have) you also have heavier gross takeoff and landing weight available than stock, by quite a bit. Need to double check but my gross takeoff is something like 4700 lbs and a hair less for landing.

I think my empty gross around 3k as well. I don't worry about it because the RSTOL kit gives you so much payload, and rock n roll short field and climb performance, it's simply not an issue.

I took off near gross only once, years ago, out of my 2665 runway, deep in a valley, on a warm summer day: Two adults, two kids, a huge afghan dog, two violins a cello, enough tiles to redo all the entire counter tops in our beach house kitchen, two 'hanging baskets' 30 lbs of dirt w my wife's flowers), our luggage, and yes, like the joke says, everything INCLUDING the (new) kitchen sink. It was HEAVY, but fine.

We recently acquired a 1968 337c and I could not get the W&B numbers to work my way. After going through the updates from years past I found a mistake - a big one. Someone miscalculated the new CG (moved wrong way) based on fuel remaining in tanks during a weighing procedure. Check your old numbers carefully :)

Weight and Balance errors
 

Some great advice here already. The only thing to add, is sometimes the scale error (if you weigh) can also add some weight. Last time I weighed an aircraft at a shop (part 145 authorized Cessna Service Center), we went through 3 sets of calibrated scales until we found ones that were accurate (at least to my liking). It was quite interesting to see the difference in weight between these calibrated electronic scales.

My '66 was never weighed at the factory - the weight was calculated.

Jeff

Reference Data
 

Just for info:
my 1966 empty weight was calculated and was 2,746.5 pounds
8 years later (1974) it was weighed after autopilot added and VOR upgraded, it was 2,886.5.

Result in 8 years, aircraft put on 140 pounds of weight. I think you all can see the trend - the aircraft are never as light as we think they are...
Jeff

Time to eat some crow...
 

Where do we keep the "crow pie" around here?

Last summer, I reported in another thread on this board that my 77 NA G model had a calculated empty weight of 2957 lbs. Folks expressed some surprise that it was so "light." I was confident in those numbers. I also wrote that we'd be weighing the aircraft this fall.

Over the years, the various mods (avionics upgrades or adds x4, VGs, gear door delete, MT props, new paint and interior x2, etc.) had resulted in a calculated BEW of 2957. I checked all the math over the 44 years of logs and it checked out.

I was certain it was going to check out closer to 2960 (or less!). Just posting to lend credence to the theory that airplanes tend to run a little heavier than their calculated weights.

Weighed it twice, to be sure, with reset on the scales in between. Followed the procedures in the service manual and POH. Readings nearly identical both times.

History:
Left the factory in late 1977 at 2992 lbs CG: 140.15 Useful load: 1638
All calculations leading up to Summer 2021 values of:
2957 lbs at 141.3 CG Useful load: 1672

Imagine my surprise when the scales settled on:
Lt Main 1285
Rt Main 1283
Nose 422

Total: 2990 CG: 139.6 Useful load: 1640

Now, this these are numbers I can have some confidence in.

Aside: jacking a skymaster is a stressful experience.

With the addition of the cargo pack, my airplane was reweighed at the last annual:
3,006 @ 140.3

The mechanic and I both did the... "that cannot be right." So, we weighed it again the next day, were careful with the setup, and came out to the same numbers.

I read about the accumulation of debris and fluid traces in the hull over time, and what looks like nothing amounts to something. Mud daubers are a big thing in many locations; we pulled about 130 lbs of mud nests out of the wings of my patrol plane. I kept one because it was so perfect!

Attachment 2651

I learned about the debris and fluid accumulation in 121 and 135 operations to include corporate jets maintained to a 121 standard. Aircraft are required to be reweighed every 36 months. No physical changes to the plane, but they always gain weight. Our 337s are +/- 50 years old.