The International Cessna 170 Association

Forgive me for the stupid question, but I was curious of the legality of modifying the cabin air/heat distribution systems in aircraft. For example, what would be required to update an older 170 with the later model heat distribution systems? Also, what about the addition of an eyeball vent on the front door post trim piece, like the new production 172/182 as in the image below? Would these be considered major airframe alterations?

Richard Pulley posted about "updating" the heating system in these forums.
The doorpost TRIM mod should be no problem because it's not structural. Non structural mods are minor alterations: a logbook entry, much of the work can be done by a pilot/owner.
BUT...don't touch the doorpost STRUCTURE one bit! It's part of the underlying and major strength of the airplane.

George,

As always, thank you for your reply. I will look for Richard's thread regarding the heat distribution system; I saw a picture of the later model firewall manifold in his airplane, but didn't recall a thread. I understand that any modifications to the front doorpost is a major alteration (I remember seeing something posted on here regarding the viability of a salvage project and it was posted that a good rule of thumb is to see if there is any damage to that bulkhead), but that begs the questions of the scope of the later-style heat distribution modification. Per the 170 book:

...Cabin heating was also improved by replacing the single outlet with a manifold-type that runs across the firewall with outlets for both front seats, a windshield defroster, and ducts to the front doorposts for heat to the rear seats...

Does this mean that the only modifications to bring an earlier model 170 (specifically a 1952) to the 1953+ heating style would be limited without essentially replacing the front doorposts? If this is covered in Richard's thread, I apologize. I am looking for it now.

Also, regarding the front post trim vent- if those were fabricated by a different material, say fiberglass, would that require a 337 or other form of approval or is it still considered a minor alteration and a logbook entry because it is a trim piece?

I appreciate your taking the time to reply.

I'm about to upgrade the heat in my '52 170.
There are quite a few threads about it on this forum, yes.

Also, there is NO doorpost or other major alteration required to accomplish this.
The side vents are in the side panels and do not affect the door post.

The changes are:
1. Newer (53 up) right exhaust and heat muff. Gets you 3" plumbing from the start and more heated air flow. I think this is optional as the older exhaust makes enough heat, its just not distributed well.
2. Newer heater valve on the firewall. This can go on the left where the existing valve is.
3. Heater distribution duct across the firewall.
4. Defroster plumbing if wanted.
5. Side panels with heat ducts if wanted (or bend up some thin aluminum to make the duct). This has nothing to do with the doorpost.

In my case, I'm going to be happy enough with the heater valve on the firewall and the distribution duct.

Here is a very well written 337 for the required work

http://www.cessna170.org/forums/viewtop ... 29&p=83266
Four Posts down
McC 337 Improve cabin heating in 52 170B (or eariler 170) Excellent descriptions and dwgs.pdf

Here's Richard's excellent posts with pics of his conversion:
http://www.cessna170.org/forums/viewtop ... ter#p51014

My IA changed the heater valve outlets from 3" to 2" since the muffler has 2" outlets and the heat muffs put out enough heat as they are. 3" hose would have motor mount clearance problems with the valve installed where it is.

Also, my IA cleared it with the Anchorage FSDO that since the existing heater was being upgraded with a Cessna designed system that was a direct replacement with no structural changes, no 337 was required, just logbook entry for minor alteration. Your results may vary.

Or just do it the easy way
http://www.sportys.com/pilotshop/product/9578

That won't do much to keep you warm tho.

and I don't think those wemac upper vents would flow as much air as the old soup cans, you get a pretty good breeze with those. I know they had to get the STC to make it easy to sell the kit but what in the wide wide world of sports would you otherwise need an STC for vents.

I have seen a rear seat vent mod that uses NACA inlets on the lower wing root fairings which duct air from under the wing thru SCAT tubing into the back seat area thru eyeball vents.

HA wrote:and I don't think those wemac upper vents would flow as much air as the old soup cans, you get a pretty good breeze with those. I know they had to get the STC to make it easy to sell the kit but what in the wide wide world of sports would you otherwise need an STC for vents.

I have flown a C-180 with this mod in the summer. I would not want them in my plane. The air flow volume is severely restricted as compared to the stock vents.

The decision to obtain an STC for after-market vents has got to be simply to avoid question of legality. (A good case may be made that the cabin ventilation system is a critical operational system becasue it affects windshield defrosting/de-fogging capability...an excellent reason to retain the original, high-output "cans" which can be directed forward, against the windshield.

The original "cans" are easily refurbished with felt seals and polished or painted to brighten their appearance. Additionally, they accomodate an Outside Air Temp gauge, negating the need to find another location or a more complicated/electric OAT system.

Rear seat ventilation is actually not bad if the "cans" are aimed alongside the upper door sill. But if individual rear seat vents are desired, it's easy to plumb into the existing wing-root fairing with scat/sceet to a pair of Wemacs* for the rear seat.


Trivia Question:
*Wemac is a brand-name...sorta like "Xerox" is for photographic-copiers. The British term for such devices was the original, but is lost in American English: "Punkah-Louvre"
Anyone know WHY?

This could be a good idea for rear seat vents.
Put them in your rear windows (with approval)

http://www.aircraftspruce.com/catalog/a ... vents3.php

hilltop170 wrote:I have seen a rear seat vent mod that uses NACA inlets on the lower wing root fairings which duct air from under the wing thru SCAT tubing into the back seat area thru eyeball vents.

Here's Cessna's solution:If it's not too wide using a NACA inlet might work with this. Here's one from A/C Spruce for a 1.5" SCAT hose--not sure what the dia. the Cessna hose is. I'd consider it a minor alteration.