The International Cessna^® 170 Association

For what it is worth, I checked with the local Anchorage FSDO on this subject. The answer I got back was if they were the steam gauges not electrical indicating things that required an STC then just put on the Cessna mast and tube as shown in one ot the IPCs, do a pitot static test and make a log book entry. NO 337 needed. Keep it simple. I have seen a lot of minor modifications, and so has the FAA that have become STC's and there was no need as there was already enough information out there to make it a simple log book entry.

Again, 337 needed at least up here maybe your local FSDO has a different take but I would just do the log entry and pitot static test and let it go at that. If you have a transponder you need that checked every 24 calandar months anyway
Cheers

When I first bought my C-170B, I had it tied down out in the open. For some reason it twice got what apparently was rain water into the pitot system which plugged off the pitot pressure and rendered the air speed indicator inoperative. I had to disconnect the pitot line and back blow it to clear it out. After the second occurence, I installed a heated pitot head, without heat, under the wing in the same locatition as on a C-172. No more problem.

I meant NO 337 needed. I have seen the same problem with the water and yes putting on the new style pitot head does make the problem go away. Although I don't think it was the static but the pitot line getting water in it. I have seen them get water so far back they freeze and split the line on the earlier open styles. Anyway just one more improvement that is worthwhile.

The REASON of importance to follow the curve of the plain pitot tube (as depicted in the Service Manual) is to prevent water from ingressing into the pitot line.

The reason a properly-shaped pitot may accumulate water despite the correct shape is.... due to the environment of the base-area and a leaking pitot line.

Here's how: A minor leak will go unnoticed in airspeed indications and a VFR airplane may never have the integrity of the pitot line checked with a proper test. Multiple flights to altitude and descents into a humid base-area-of-operations will (coupled with minor pitot-line-leak) allow the entrance of humid air (not merely impact-air, which normally results in increased-pressure ONLY)... and just like inside your fuel tanks...the humid air will condense either during descent...or during overnight storage when the atmosphere cools off.
The original system (and most systems modified with heated pitots) fail to include pitot-line SUMPs and DRAINs....so the water builds up until it blocks the system.

Anytime the pitot system is modified... it's a good idea to incorporate ALL the later developments including pitot-line sumps and drains. Get it ALL approved at one swell foop.