Airspeed Indicator Issues

How to keep the Cessna 170 flying and airworthy.

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danparm
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Joined: Tue May 26, 2015 3:00 am

Airspeed Indicator Issues

Post by danparm »

Had a rough annual this year and still feeling the effects from it. Had to replace a cylinder, which I doubt added a lot of horsepower AND a STOL kit. I'm now taking off and climbing at 1000 fpm while indicating ~60 mph.

The 91.411 and 91.413 tests were due this year at annual time, so I had the avionics guy do them. He noted that there were several leaks in the pitot/static system that had to be fixed. First flight after the annual the airspeed indicator was barely off the peg when I raised the tail. I rotated barely indicating 40 mph. My AOA was showing a full green donut as it usually does and 1349D was climbing normally. At 2400-2500 rpm level at 3000’ I was only showing ~90 mph when I’m usually closer to 110 mph.

Had the avionics guy take a look at it. He checked the angle of the pitot tube against the template. Said the angle was off a small amount. I still got the same results, about 20 mph low regardless of attitude and phase of flight.

Then at the beginning of August on takeoff the indicator face filled with moisture and the needle never came off the peg. After giving it a tap it stuck at just above 60. Had the indicator removed, avionics guy said there was evidence of a puddle of water at the bottom of the indicator. It was repaired, reinstalled, and flown a couple days ago. A sump bottle was also added. Same result, about 20 mph low.

I attached a picture from the flight. Calm winds were reported at the field, windsock agreed. Winds aloft forecast was interpolated for 3 kts of wind at 3000’.

The previous Part 91 tests 4 years ago were completed without issue, no leaks reported. The aircraft is hangered in Central Virginia. Prior to the annual it hadn’t been flown in visible moisture for a long time, quite possibly over a year even two. I couldn’t remember. In July, a month after the annual, it did sit outside for a couple days where a shower may have passed by. I don’t know how that much moisture could’ve been introduced into the system.

Does the brain trust know of any intricacies of the 1951 170A pitot/static system? What questions and troubleshooting should I be asking and doing at this point? Planning on taking it to the shop that did the previous tests 4 years ago. Potentially another if they’re booked up.
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n2582d
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Re: Airspeed Indicator Issues

Post by n2582d »

Dan,
Check out the last entries on page one in this thread. A 1/4" cap with a "Remove Before Flight" streamer on your pitot tube might help prevent water ingress in the future.
Gary
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GAHorn
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Re: Airspeed Indicator Issues

Post by GAHorn »

We (Jamie and I) parked N146YS on the ramp at Vicksburg a few years ago and it rained that week. I'd heard of water in the pitot line from other Members and, frankly... I was skeptical of the stories completeness... but on the takeoff next made from Vicksburg to Ft. Smith, AR... on the takeoff roll I noticed the airspeed was unresponsive.
I glanced out at the pitot's "flip-cover" and saw it was opened by the relative wind...but still no airspeed indication.
As the airplane was clearly above flying speed I allowed it to leave terra firma and during the initial climb the airspeed suddenly indicated but waay-below normal climb speed for the pitch attitude.


Not wishing to work on it in Vicksburg I continued toward Ft. Smith and watched the airspeed needle "dance" a bit for almost an hour...then settle down to normal indications. It's worked fine ever since, and I suspect water entered the OEM-configured/shaped pitot tube while tied down in the 3-point attitude and that eventually the bubble of water in the curvature of that tube dried out in-flight. I also suspect that the "flip-cover" may have actually contributed to the problem by the little flap conducting water into the tube in capillary-fashion.


I've never observed it before or since. (I'm usually hangared but have been tied down in rain numerous times over the last 20 years I've owned her.)

Note that SK 175-8 is applicable to the 150/172/175 aircraft only, despite the similarity to the 170. The sump bottle is available very inexpensively from Spruce. (The 172 we bought last year had a cracked sump and Spruce sold the genuine Cessna replacement for $8.)
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons. ;)
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danparm
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Joined: Tue May 26, 2015 3:00 am

Re: Airspeed Indicator Issues

Post by danparm »

n2582d wrote:Dan,
Check out the last entries on page one in this thread. A 1/4" cap with a "Remove Before Flight" streamer on your pitot tube might help prevent water ingress in the future.
Thanks Gary, I've got some heavy duty shrink sleeving hanging on the wall that I think will work nicely, and I've got a "Remove Before Flight" streamer that came with an aviation related shipment some time ago. I already use one on my key fob lol.
gahorn wrote:We (Jamie and I) parked N146YS on the ramp at Vicksburg a few years ago and it rained that week. I'd heard of water in the pitot line from other Members and, frankly... I was skeptical of the stories completeness... but on the takeoff next made from Vicksburg to Ft. Smith, AR... on the takeoff roll I noticed the airspeed was unresponsive.
I glanced out at the pitot's "flip-cover" and saw it was opened by the relative wind...but still no airspeed indication.
As the airplane was clearly above flying speed I allowed it to leave terra firma and during the initial climb the airspeed suddenly indicated but waay-below normal climb speed for the pitch attitude.


Not wishing to work on it in Vicksburg I continued toward Ft. Smith and watched the airspeed needle "dance" a bit for almost an hour...then settle down to normal indications. It's worked fine ever since, and I suspect water entered the OEM-configured/shaped pitot tube while tied down in the 3-point attitude and that eventually the bubble of water in the curvature of that tube dried out in-flight. I also suspect that the "flip-cover" may have actually contributed to the problem by the little flap conducting water into the tube in capillary-fashion.


I've never observed it before or since. (I'm usually hangared but have been tied down in rain numerous times over the last 20 years I've owned her.)

Note that SK 175-8 is applicable to the 150/172/175 aircraft only, despite the similarity to the 170. The sump bottle is available very inexpensively from Spruce. (The 172 we bought last year had a cracked sump and Spruce sold the genuine Cessna replacement for $8.)
The avionics guy asked me if I wanted a sump bottle installed, just in case. I had him do it, super cheap. He said he blew out the lines before re-installing the indicator. I'm not getting any erratic indication. The needle moves smooth but just 20 kts low. It's barely off the peg when I raise the tail and near 40 on rotation. Doesn't look like I'll be going anywhere with it this week for another opinion. My other ride is an Embraer and that pays the bills, along with all the rain we are supposed to get with Florence on the way.
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GAHorn
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Re: Airspeed Indicator Issues

Post by GAHorn »

One simple, inexpensive method to test your pitot (inlet) system for integrity is to use surgical tubing. Slip it over the pitot tube, then close the opposite end of the surgical tubing by pinching it, then "roll" the tubing upon itself very slowly while observing the airspeed indications. A leaking pitot inlet system will allow the airspeed indicator's indications to decay as the inlet pressure leaks out.

You obviously have already had your airspeed indicator overhauled/calibrated?

You can build a simple manometer and, using a "T", plumb it into the surgical tubing pressure to see what your airspeed indicator reads at the appropriate water column height. A water column height of 6.5397 inches on the manometer equals .2362 psi and 100 KNOTS of airspeed. (100 KTS = 115 MPH)
manometer.pdf
How to build a simple Manometer
(80.1 KiB) Downloaded 749 times
WARNING: Many manometer measurements are made using Mercury (Hg) rather than water. Don't let other uses of your device inadvertently use the wrong liquid measure.
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons. ;)
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