Radio shelf weight limit

[Freightpilot27]

OK, so I don't have a 170, But my 172 is a family heirloom and I can't afford 2 airplanes, otherwise I would.
I'm hoping someone in this forum will have the info I’m looking for, or know where I can find it...
Question-
I would like to know if there is a published max weight (and what it is) for the radio rack shelf behind the baggage compartment of the early 172's (I have a '63 D model). I have looked in the Service and parts manuals as well as the Electronics manual. I can find everything else I need except for the max allowable weight (if there is one).

Thanks,
Bob

[GAHorn]

Bob, I've checked my copies of the IPC and the SM (Ilustrated Parts Catalog and Service Manual) for your aircraft and cannot find any reference to a radio shelf behind the baggage compartment, therefore I suspect it was not a standard item for that model aircraft.

It was likely installed as a part of an earlier radio installation, probably for a power supply for an older vacuum-tube radio. If so, that shelf would have been either part of an installation-kit for a particular radio, or it would have been manufactured by the installer. Either way, the radio installation would have required that whatever piece of equipment was placed upon it should meet the requirements of applicable FARs (CARs) and that likely would have required a 9-G load. So if it was a 2.2 lb pwr supply then the installer or mfr'r of the rack would have to determine that it would support such a load.

It's likely the old radio was removed and the rack/shelf was left insitu. If you wish to utilize it for a new installation, you (or your installer) will have to determine it's suitability for the purpose.

Keep in mind that there are two separate weight problems with which you are faced: The support-capability of that shelf..... and the affect of whatever weight you place upon it on the aircraft's C.G.

[lowNslow]

I also have this shelf installed in my 170B. I believe it was used for the original Lear radio, I now use it for my ELT. I don't know of any weight restriction published for this other the checking the CG of the aircraft after you install the new equipment. I suppose you could also make the argument that you would have to include the weight in the 120 lbs. limit for the aft cargo area.

[GAHorn]

Thru the years Cessna offered "standard" radio-installation packages to new aircraft purchasers. You can see evidence of this in most early Owner's Manuals. Those standard packages had installation kits such as I mentioned previously, so if Cessna sold 600 A-models in 1950, it's likely that 400 of them had the standard radio installation.... but that would not be an item to be illustrated in an IPC... it would likely be in an "Electrical Equipment" catalog or document which is a separate matter.
Alternatively, an owner might purchase a new Cessna and a local avionics shop might make an installation. That shop might either purchase the same kit from Cessna, or perhaps might be a Narco dealer or G.E. dealer, etc., and have access to those mfr'r installation kits.... OR.... the shop might simply manufacture a custom shelf for the specific installation. Either way, whoever designed/constructed/installed the shelf would be responsible to determine suitability to support the weight per CAR's.
The 120 lb limit for the baggage compartment would not only take into consideration the C.G. limits of the aircraft.... but the weight-capacity of the baggage compartment FLOOR. Therefore an added radio-shelf aft of the baggage compartment would have little or nothing to do with the limit placed upon the baggage compartment.
(In all cases, the pilot would need the info, however, in order to compute CG for flight. The Form 337 or the WT/Bal sheet and/or equipt list should be so noted.)

[Freightpilot27]

Thank You gentlemen,

I don't know about the 170's But I do know in '63 (and subsequent years) all 172, 180, 182, 185, 205, 206's came with the radio shelf’s in various locations. In our's it was originally used for Lear and later on Narco radio power supply's and GS receivers. Many moons ago I replaced the antiquated Narco MK12A radios with new King KX155 & 165's (Along with way too many dollars worth of GPS's and an Autopilot) eliminating any radio equipment on the shelf. This shelf is part of the aircraft structure and was included in the airframe from the factory.

The Parts manual for my model year show the self, corresponding bulkheads and hardware.
My Electronics Installations & Service/parts manual include drawings of the shelf including the installation and weights of various individual radio components available in the 60's.

I am working on an extended baggage installation via field approval, and was hoping to minimize the engineering involved if I could use the maximum allowable weight of the radio shelf as the max baggage weight in that area.

Thanks,
Bob

[cessna170bdriver]

I also have this shelf installed in my 170B. I believe it was used for the original Lear radio, I now use it for my ELT. I don't know of any weight restriction published for this other the checking the CG of the aircraft after you install the new equipment. I suppose you could also make the argument that you would have to include the weight in the 120 lbs. limit for the aft cargo area.

Karl,

I think the 120 lb limit for the baggage area is a structural consideration for the compartment floor, not a CG limitation (you can put a LOT more than 120lb in the baggage area and keep the airplane in the CG envelope). If the radio shelf is installed behind the baggage compartment floor, I don't think it would deduct from the 120 lb allowable baggage. Anyone please feel free to tell me why this would not be the case.

My 55 B-model has a shelf back there that held the power supply/transmitter for the Narco Mk12A nav/com that was installed in the airplane when I bought it. That shelf now has the ELT installed on it.

Miles

[lowNslow]

If the radio shelf is installed behind the baggage compartment floor, I don't think it would deduct from the 120 lb allowable baggage. Anyone please feel free to tell me why this would not be the case.
Miles

Miles, I only brought the possibility up because mine is installed basically as an aft extension of the floor down one side behind the bulkhead. But you are correct, it should not be an CG issue.

[cessna170bdriver]

Miles, I only brought the possibility up because mine is installed basically as an aft extension of the floor down one side behind the bulkhead. But you are correct, it should not be an CG issue.

You have a point there; it might take some engineering to figure that one out. I can't visualize how mine is installed; I don't remember whether it's attached to aft baggage bulkhead or not.

Miles