Screws on the port side of C-172

[tmevo6]

Can anyone please tell me what are the purpose of the 2 screws located on the port side, aft of the luggage for?

I understood that they are for the measurement of weight and balance, but how can they use the pair of screw to do that?

Mant thanks in advance.

[411A]

These are generally used on Cessna aircraft to adjust the landing gear to level the aircraft, in preparation for weighing same, among other things.
A special tool is attached by the screws for this purpose, and that tool includes a bubble level and, on several twin Cessna models, an inclinometer.

[GotTheTshirt]

For adjusting the landing gear ??:eek:
Are they very long screws ??:)

[411A]

>>Are they very long screws ??<<

No, but very strong...:E

The screws hold the aforementioned fixture, which contains the level, so that the landing gear can be adjusted correctly.

[barit1]

Every aircraft has a defined method for leveling it when conducting an empty CG check (done with scales under each wheel). The weight measurements don't mean anything unless the plane is level.

[john_tullamarine]

If you consider the picture with an aircraft on scales .. the CG position projected to the horizontal axis will be dependent on the attitude of the aircraft .. if nose down, the apparent CG is further forward, if tail down, further aft.

To make sense of the numbers, CG data needs to be repeatable and this requires the numbers to be referred to some reference attitude. This level position is defined by the OEM and is significant only as a rigging reference.

All aircraft are required by the Design Standards to have a means for checking the level reference for weighing.

Typical means are

... put a spirit level

(a) on two screws on the side of the aircraft

(b) on the top of the tail cone

(c) on the seat track

.. etc

or ... drop a plumb line

(a) to pass two screws mounted vertically on the side of the aircraft

(b) from a jig point to an inscribed plate

.. etc.

There may be other variants but the techniques listed above cover the great majority. Sometimes for tailwheel aircraft the OEM may schedule a taildown weighing method but this is far less satisfactory as it presumes a vertical location for the CG for the trigonometry necessary to convert the measured CG to a presumed level CG.

There will also be a means declared to achieve a lateral level .. especially important on rotary machines.

The weighing will be done either with

(a) weigh bridge style platforms (not common)

(b) glorified bathroom scales (but much more accurate, repeatable, and expensive) for smaller aircraft. Typically these are either strain gauged beam construction or mechanical balances.

(c) compression disc load cells which sit on top of the jacks and beneath the aircraft jackpad fittings. For smaller aircraft the aircraft is then jacked clear of the floor .. best if the aircraft started off level otherwise one can see some dreadful side load scale errors. For jumbo size birds, some hangars are set up to have the jacks remain stationary while the floor area under the wheel trucks is lowered clear of the wheels. There have been some hydraulic disc cells but of limited success in the marketplace.

And every now and then some clown rebuilds an aircraft ... losing the level jigs in the process .. I have only had this once .. turned up to weigh a C402 (as I recall) and found no jig references in the airframe .. which had had a major rebuild. The maintainer concerned hadn't bothered to find out what the bits he discarded were intended for ... As I recall we wasted quite some time with another example and the manuals working out an equivalent jig for the weigh.

[GotTheTshirt]

John,
The evolution of weighing is quite interesting.
When I was a lad ( a few years ago now!) weighing was done with your glorifed bathroom scales.:}
Then came the magic of load cells :)
Most people changed to this. I have seen a couple of nasty accidents where the aircraft tipped and shot the load cell out like a bullet ! and also a jack through the wing.:uhoh:
If you have been involved with weighing aircraft for any length of time you may remember Ken from Aviation Traders who used to come round with his station wagon and perform his load cell magic before your very eyes.

The last few large transport aircraft I have been involved with weighing have gone back to bathroom scales !
The full circle !:}

PS the FAA TCDS usually shows the levelling method