What factors should I consider regarding weighing my plane?
Search strings: aircraft weighing, weight and balance, re-weighing, aft ballast weight
Technical Editor:
The Shop Manual and the POH both contain instructions related to weighing the aircraft; as do some of the many Service Bulletins, etc. What follows is a mix of fact and opinion/preference; you’ll be able to tell them apart. I have not repeated the basics (indoor weighing, jack pads, tail-stand usage, etc.) that are spelled out in the Shop Manual.
The aft ballast weight makes a significant difference in the plane’s empty CG number (for the better); it usually moves the empty CG aft by at least one inch (and that’s a lot). If you wish to do a pre-installation estimate for your plane, the Fuselage Station for the weight is FS-288. The plane needs to be properly re-weighed, if the ballast weight is being installed. That means weighing it with scales that can mate with the jack pads; or having the gear blocked so that they are fully extended during weighing. If the plane is weighed with the weight on the wheels, in the normally parked position, the weighing will not be valid because of the trailing link gear. While Beech factory-installed the aft ballast weight only in certain Sports and Sundowners, it has been field-installed in many Model 23s to correct forward CG conditions. There is at least one Form 337 in BAC Downloads that can be referenced for a field install. With the Beech part being used, and being installed as specified by Beech (a diagram is in the 337 download), all that is needed is an informational 337 sent for file by your A&P-IA. Some have been installed with a logbook entry only; but getting a significant W&B change like this documented on a 337 is probably a better bet. On the other hand, if the job is properly done; and the plane is properly re-weighed with an accurate new W&B; it is highly improbable that it would ever become an issue (due to the Beech part and the precedent-setting factory installations).
You should get everything readily removable out of the plane for the new weighing. Tools, flight bag, maps, hand-helds and mounts, accessories, tow-bar, everything. I also much prefer fully drained tanks and a full oil load in the engine. Your tech may tell you that it is easier to weigh it with full fuel, then subtract usable fuel. While true in principle, I have seen it lead to many errors in the final numbers. This is especially true if the plane’s records contain an artificially high (legal but invalid) number for unusable fuel. For example, my Sierra’s REAL unusable fuel is between a quarter and half-gallon. Most of the aircraft have unusable fuel listed as more than a gallon per side; some have it as high as four gallons per side. This one factor can adversely affect the final real payload number by as much as 50 pounds on some of the planes.
This is also the time to install a lightweight starter and alternator, if you ever intend to do it. Ditto for any propeller changes, whether an overhaul or replacement. An overhauled fixed-pitch prop will come back lighter. A 3-blade McCauley replacing a 2-blade Hartzell will add weight.You might consider installing the lighter and lower-drag 15:00x6x6 tires; and installing the lighter-weight 124J hoses firewall-forward, in place of the firesleeved standard hoses.
If you get the plane as empty as you can, including unused avionics removed (along with related cables and antennas), then you will wind up with a much more ‘real’ empty weight. That gives you a much more realistic payload number. You can always weigh what gets put back in the plane for normal flying. But by having a real empty weight, you can readily make adjustments in the nature of the payload, when needed. For example, leaving the heavy map bag behind, for a short burger run with four big adults on board. Most of the planes also have an 8-pound taxi fuel allowance, that is in addition to the calculated payload. In other words, you can legally taxi out while being 8 pounds over your normal max gross weight.
Here is another example, if you have the readily removable separate 3rd-4th seats. The seats will be listed in the current equipment list, with their weights noted (usually 12 pounds each). You might want to consider doing the basic W&B with the rear seats removed, to boost the empty payload number. That makes it easy to add the 12 pounds (or whatever yours are) to the passenger weight when the seats are needed. Some examiners won’t let you fly with the rear seats out, if they are included in the basic empty weight; even though it is easy to subtract their weights (and the split rear seats are made to be readily removable). If the plane is weighed without them, then there can be no issue; they are payload when they go back in, just like the rear seat passengers, and with the same arm.
I highly advise that the tanks be drained empty. The normal method is to fill the tanks, then reverse-engineer to a valid empty weight. This frequently introduces errors. The idea is to obtain the absolute lowest legal empty weight that you can. That way you have a maximum legal payload, which you can use for whatever you wish. Another benefit is that you can fill the tanks, and find out what they really hold. If you wish, you can crack the fitting on one end of each of the 3/8″ fuel hoses at the wing root. Add fuel to each tank one quart at a time, until you see fuel seeping out of each side. My usual experience is that the actual unusable fuel is three pints; 1.5 quarts. If you let the crew fill the tanks for weighing, they will subtract the unusable fuel in the POH; that can be as much as six gallons. That will result in an incorrect payload reduction of at least 30 pounds per side, or 60 pounds total. You don’t want to give up that 60 pounds of legal payload; even if you do wind up always using it for fuel.
Once an artificially high empty weight goes into the new W&B, with the concurrent lower payload, that unnecessarily low payload is in the records until the next weighing. That can affect resale, FAA/NTSB conclusions after any accident or incident, etc.
And finally, I need to re-emphasize this key point. THESE PLANES ARE OFTEN IMPROPERLY WEIGHED, DUE TO THE TRAILING-LINK GEAR. They must be weighed on scale jacks, or with standard jacks resting on scales. The tail is connected to a scale that has been weighted with a few hundred pounds. The plane is jacked up using the scale jacks, until the nose is clear of the ground. The negative weight (weight loss) on the tail is used in the calculations, rather than nose wheel weight. If the planes are weighed in the conventional manner, with the wheels on scales and the gear cushions partially compressed, the empty CG will incorrectly come out too far forward. On a typical Sundowner there can be a three-inch difference in main gear contact-patch position, between the gear extended and the weight on the gear. In part, this is why Beech specifies an initial installed height for new donuts; that setting is what determines the gear extension for weighing, when the gear must be blocked down for weighing.