Poor Defroster Air Flow - any experience

[Dave Underwood]

Has anyone had poor air flow out of the defroster vents on a later G model.

The problem is one of what seems to be low levels of air flow out of the defroster vents, even when we set the controls and close all the other vents on the floor. There always seems to be a disportionate amount of air coming out of the floor vent on the right of the console as well as those on the floor that you can not shut off.

We have looked at the diverter valve behind the right pedal and see one flap valve which controls air to the thermostat and thus the cycling of the heater and another that is supposed to control warm air to the defrost. The defrost valve appears to operate backwards to expectations, but does open and close the outlet that leads to the scatt tube and then upwards to the top of the panel.

It would appear that when the defrost lever is in the down and on position, that its valve is actually partially closed. When you put the control in the upper position showing defroster off, the valve is actually fully open. It would seem to be backwards although everything is rigged per the diagram in the service manual.

Another question is, am I being unrealistic expecting a strong flow through the defroster vents? Please, no comments about Cessna engineering and Design.

Has anyone taken this area apart or looked closely at how it works and how the air is supposd to flow?

Regards - Dave

[kevin]

I've never looked at the design, can't help there, but there may be something wrong with your setup, as I got plenty of defrost flow on my '73 P337. I live in a humid area very prone to condensation, so it HAD to work well for me.

Only problem I ever had was during maintenance, mechanics (embarrassingly including me) would disconnect the duct that bring air up to the windsheild on the right side, and forget to reconnect it, leaving the right side without any airflow at all.

Sorry I can't be more help.

Kevin

I had the same problem as Kevin on my FIRST flight up north a few years ago. Damn cold with no heat at 10,000' in December!!! It was an improperly connected duct, and an unpleased spousal unit and toddler contingency - if you hear what I'm saying.

Since then, I've not had any problems, and always fly with blankets and a space heater as back ups - but have not used them in necessity . . . .

I believe the true experts on this should be, at a minimum, Larry and Gerry.

Gents? Any problems or suggestions?

Keven
________
RHODE ISLAND MEDICAL MARIJUANA DISPENSARY

[hharney]

NO PROBLEMS WITH MINE. I NEVER WEAR A JACKET BECAUSE THE HEATER WORKS SO WELL. I'VE HAD OAT NEAR -50 WITHOUT PROBLEMS. I'VE NEVER HAD TO MONKEY WITH THE DUCTS.

[WebMaster]

Check the ducting. disassemble it. Look for bad connections. It is not a big deal to take it appart and put it back together. When you do, make sure that all the hoses are clear. It is not impossible for someone to have stuffed something inside the ducts. I say this, because my plane is in annual right now, and the IA said they found a wad of insulation stuffed up inside the outflow duct. That of course affects the airflow throughout the cabin.

[Don Hickman]

Well, I'm glad someone has finally raised this issue, but the responses haven't helped much! My stupid defroster has NEVER worked properly, at least to my thinking. It won't clear more than a tiny space at the bottom of the windshield, no matter what the settings.

I've complained to my mechanic for years now about it and he says that all the hoses are clear and properly connected. Big deal! Stupid thing still won't work.

In the winter, as long as I run the heater, the windshield stays reasonably clear. But without the heater, the defroster itself does nothing. And there is no discernible airflow at the windshield outlets.

Since the hoses are connected and clear, I'm at a loss to explain why there is no appreciable airflow at the windshield outlets. The floor vents put out lots of hot air.

[Dave Underwood]

Don, my experiences so far match yours. Lots of flow through the floor vents but poor to the screen.

The following is what have we found out so far having had the whole thing apart and having spent a fair bit of time on figure 90 in the parts book and figure 13-2 in the service manual.

When we put airflow output through the Y connector, we get moderate flow to the screen, not great, but passable flow levels. I would have expected a good blast, but that is not what we get. There are no leaks on either of the defrosters outlet units that we can find so the only thing we figure is that the large chamber below the vent openings slows the flow down a lot. We are going to check for more leaks just the same as it does sound to me that there is air leakage under the glare panel and behind the instruments.

We did find that the system seems to work backwards. That is the valve to the defoster although rigged per the book, operates backwards. When the control is in the up and off position, the valve is open. When the control is in the down and open position, the valve is mostly closed. We have checked and it is almost like the split pin hole that holds the control arm was drilled 90 degrees wrong on the shaft that holds the valve plate or the valve plate holes are out by 90 degrees.

The other control to the flow box behind the right P1 pedal controls the amount of air flow to the thermostat and thus we assume the rate which the heater cycles on and off. Its bracket means the rigging on the defroster flow diverter can only be set up as shown in the manual.

We did find some skat the was a bit ratty and now replaced and the rectangluar duct under the P1 floor was bent inwards meaning there was air flow loss under the floor.

We are now putting it all back together and will try it again, but this time with the various floor heater outlets turned down a bit. Hopefully that will give us reasonable flow through the defroster vents.

The P2 heater vent seems to flow the best of all, so perhaps limiting its flow might also help. I never sit there so I won't get cold feet.

I will let you know what else we find.

Regards - Dave

[Dave Underwood]

An update.

We disconnected the return flow scatt tube from the firewall to the alu pipe that then runs to the heater and connected a vacum, blowing into the heater area. We got pretty good airflow through the entire front air system as a result.

We got some flow through the defrosters, but could also hear air escaping. Removed the AI and we could then see and feel a significant air leak between the lower plate and its mounting surface in the front side, just to the right and above where the scat mounts as you look at the setup through the AI hole.

For reference, in my parts book it is item 89 in figure 90, Cabin Heater & Defroster Installation.

We have tighened things up and sealed the area and it seems much better now.

We are also going to run with the floor vents partially turned off and see how it goes in the air.

In the process we also found a major pressurization leak due to the poor fit of the gaskets on the heating system intercooler, both top and bottom. Good news as the pressurization system was becoming a pain!

Regards - Dave

[SkyKing]

Dave,

I think you lost some of us in the translation. What's the "AI" you're talking about?

Also, regarding the Scat tubing, you might want to check the circumference of all the runs to make sure there aren't any worn spots or holes in the tubing, especially where there is flexing or rubbing on other surfaces. The RED High-Temp RTV is excellent for maintaining the integrity of the Scat to keep the fiber wrap in place and also to cover any thin spots in the covering. Also be aware that if you have to replace any Scat to select the proper replacement type.

Regarding the Janitrol heater, has it been tested or replaced per the recent AD? You could have a hole in the plenum that would allow pressurized air to escape.

One other thing, you've never mentioned if the blower-fan above the pilot's rudder pedals is functioning properly, or if the Scat is hooked up right.

SkyKing

[Dave Underwood]

Skyking, thanks for your note.

The AI is the Attitude Indicator.

The heater is tested per the AD and works great with no leaks.

The hoses out of the blower were checked and in fact replaced last year due to ratty appearance. No further leakage.

RTV is great! How did people exist, let alone maintain airplanes before it was invented. But then again they had duct tape, oh well.

With the various leaks and hoses fixed or replaced, hopefully things will be better now on both the defroster and pressurisation front.

I am travelling next week and will fly it when I get back, hopefully with good news on all the issues on the squawk list.

Regards - Dave