Cessna Pitot Tube ?

n2o2diver

New Member
Can someone very familiar with Cessna Singles tell me if there is a drain hole in the PITOT TUBE/MAST/HEAD ???

I am an Avionics Tech and I can say that any hole in a pitot line has always produced an airspeed problem.

So it just doesn't make any sense that there is a drain hole (I should say an un-capped drain), seems there would never be any build up of impact air if it was all just going out a hole. Now some people have told me that it is a calibrated hole and I just don't buy it. The VSI has a calibrated hole and it always goes back to zero, so would your airspeed indicator.

Any takers?
 

Alchemy

Partner, Ally, Friend
Yeah, there is a drain hole. You always have to check that it's clear of debris during the preflight, just like you do the primary pitot opening. The mast-type pitot sources on pipers also seem to have a drain hole. I'm not sure of the physics behind how it works, but I'm pretty sure that there is indeed a drain hole on the bottom of those pitot tubes.
 

n2o2diver

New Member
Pipers, contrary to popular belief, do not have an open drain hole in the mast head. Both of the holes toward the back and bottom are STATIC PORTS! On some Pipers those hole are there but they are NOT used, because there are fuselage mounted static ports which are used instead. Piper drains are in the cabin, near the floor by the pilots left. You push them to drain out moisture and when release they seal close.

Can someone run out to a Cessna and blow in the pitot tube and tell me if any air comes out the hole?
 

E_Dawg

Moderator
Has anyone else noticed that hole thing on top of the extention of the pitot tube on the 152s? What is that?
 

Alchemy

Partner, Ally, Friend
yeah, I remember my instructor mentioning that the hole on the back of the piper pitot mast was a static port but I thought he was wrong since the plane I was flying also had "traditional" style static ports mounted on both sides of the fuselage, and the Pilot's operating handbook specifically mentioned that the static ports were on the sides of the fuselage. Since it wasn't a static port I just assumed it was a drain like the cessnas. I'm also aware of the pitot-static drain valve in the cabin. Guess my CFI was partially right after all. I must be flying one of the planes where the pitot-mast staic ports are not used.

About blowing into the pitot tubes; that is one of the things we are told specifically not to do as student pilots, and we're warned that doing so can result in damage to the pitot system. As an avionics tech you are probably much more qualified than I am to know wether doing this will actually cause damage or not.....
 

n2o2diver

New Member
Your right blowing in the tube is not suppose to be good for it. I have seen it being done for 12 years and really haven't seen it cause a problem. Sometimes we used to use a turkey baster rubber ball or the one off of a blood pressure wigit to pressure it up and it would hold the pressure because it fit tight around the front of the tube, we could then run around and look in the cockpit to see that both indicators read the same and compare them to the display in the HUD, reason again to believe there is NO DRAIN, if there was the air would just bleed out.

I guess I'll just have to get one and cut it in half, anyone want to volunteer theirs in the name of science?

We have a piper style one cut in half already!
 

Alchemy

Partner, Ally, Friend
Well, I'll take a closer look for you when I go out to the airport tomorrow morning, maybe lightly blow into one and see if I can feel any air flow coming out the drain. Will let you know tomorrow night.
 

Alchemy

Partner, Ally, Friend
I checked a few cessna 172's on the flight line today and made sure they all had a drain hole in the pitot tube. It's a very small hole, about 6" behind the main opening, pointing sort of diagonally down/backwards. It's so small you couldn't even fit a shoe string in it, probably about the same width as the metal a paper clip is made out of.

I tried blowing gently into one of the tubes and could not feel any air coming out of the drain hole. For one, it was a pretty windy day. Also, the drain hole is so small and so far away from the primary pitot opening I don't know how much air you could force out with your breath. Hope this helps.
 

PhotoPilot

New Member
Completely off topic reply, but . . .

n2o2diver - where do you dive that calls for mixed gas? We do a bit of mixed gas up here in Lake Superior when on deep wrecks. Darn cold down there though . . .


Back to the topic at hand!

-PhotoPilot
 

pscraig

Well-Known Member
There are some nice dive sites off Sebastian and especially Palm Beach, FL. You're only at 60-70 feet but the nitrox allows a nice increase in bottom time. n2o2diver is out of Vero so I've probably run into him unknowingly.
 

n2o2diver

New Member
All dives are mixed gas, technically!

Mostly some deep wrecks off of Catalina and some off the West Coast of Australia.
 

averyrm

Well-Known Member
[ QUOTE ]
Now some people have told me that it is a calibrated hole and I just don't buy it. The VSI has a calibrated hole and it always goes back to zero, so would your airspeed indicator.


[/ QUOTE ]

The way it works as a calibrated hole is because there is always pressure coming in from the front. In the VSI, the pressure comes to equallibrium. When you climb, the pressure is greater in the instrument than out, but when you level out the pressures equalize because of the hole.

With a constant supply of ram are coming in the front of the pitot tube, and a smaller drain hole constantly leaking out air, some air is forced into the pitot-static system and some leaks out the drain hole.

Hope that helps
 

JHines

New Member
The outer probe you see is a shell which surrounds an inner tube that conveys stagnation (ram) pressure to the ASI. The inlet of the inner tube is set back from the probe ram opening, so the probe drain hole (yes, it's a drain) is in fluid communication with the inner tube, but there's no leakage of ram air once it enters the inner tube. There are different ASI failure modes depending on what hole ices up - if the ram inlet on the probe ices up, the ASI will see static pressure only (coming in the drain hole) and the ASI will drop to zero. If the ram inlet and the drain freeze, the pressure at the moment of freezing is trapped and the ASI will "act like an altimeter". Sorry, can't find a Cessna diagram. FAA Instrument Flight Handbook FAA-H-8083-15 has a diagram of a more complex probe, but principle is the same.
 

E_Dawg

Moderator
[ QUOTE ]
The inlet of the inner tube is set back from the probe ram opening, so the probe drain hole (yes, it's a drain) is in fluid communication with the inner tube, but there's no leakage of ram air once it enters the inner tube.

[/ QUOTE ]

 

EatSleepFly

Well-Known Member
What he means by "fluid communication" is that there is no difference in pressure between the outer and inner tubes. The inner tube is set back from the probe opening so that water entering the opening is more likely to end up in the outer tube, and thence out the drain.
 

JHines

New Member
EatSleepFly - thanks for the clarification, that's exactly what I meant. Dang patent lawyer language slipping into my postings.
 

sixpack

New Member
[ QUOTE ]
Can someone run out to a Cessna and blow in the pitot tube and tell me if any air comes out the hole?

[/ QUOTE ]

Two very important things to consider before blowing into a Pitot Tube.

1. Make sure the pitot heat is not on.
2. Make sure none of your friends are watching.
 
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