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pitot static blocked

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pitot static blocked

Old 6th Mar 2019, 19:58
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pitot static blocked

Hi Guys

Wondering can anyone share something on this that can help me remember easier regarding questions about the pitot or static blocked in a decent or climb etc etc

Thanks in advance
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Old 6th Mar 2019, 20:01
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Try this, it has worked for me in the past. Understand how the instruments work. Good luck.
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Old 6th Mar 2019, 20:29
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Try a search on "pitot tube blocked".
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Old 6th Mar 2019, 20:49
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pT = pS + pD
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Old 6th Mar 2019, 21:06
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Old 7th Mar 2019, 00:35
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I remember this one from flight school.

Effect on ASI with pitot or static blocked: POCUD & SUCOD

Pitot
Over read
Climb
Under read
Descent

Static
Under read
Climb
Over read
Descent
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Old 7th Mar 2019, 03:02
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https://en.wikipedia.org/wiki/Pitot-static_system
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Old 7th Mar 2019, 12:47
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Originally Posted by Feather44 View Post
I remember this one from flight school.

Effect on ASI with pitot or static blocked: POCUD & SUCOD
Why is the drain hole not considered in these rules of thumb?

If the pitot is blocked but its drain hole is open - ASI falls to zero.

What happens if the drain is blocked but the pitot open? Presumably nothing changes until the pitot also gets blocked when you have a POCUD situation.

What typically happens in real life? I might guess that in pitot freezing situations the drain hole blocks first then the pitot hole itself?
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Old 7th Mar 2019, 14:03
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Because the poster is asking for something EASY to remember.
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Old 7th Mar 2019, 14:47
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May I suggest

PUDSOD

Pitot blocked
Under read
Descent
Static blocked
Over read
Descent

and vice versa for climb.
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Old 7th Mar 2019, 15:40
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Originally Posted by double_barrel View Post
Why is the drain hole not considered in these rules of thumb?

If the pitot is blocked but its drain hole is open - ASI falls to zero.

What happens if the drain is blocked but the pitot open? Presumably nothing changes until the pitot also gets blocked when you have a POCUD situation.

What typically happens in real life? I might guess that in pitot freezing situations the drain hole blocks first then the pitot hole itself?
Good question. I am aware of a handful of events where the pitot has been blocked and pitot pressure has drained to almost match static due to drain hole being open. As you suggest, plugging just the drain hole would not have an effect unless flying into precipitation in which case one would expect the pitot itself to plug up shortly after or at least to have the pressure reading affected by water accumulating inside the sensor. Maybe some pitot probe experts can comment on the relative heating provided to the drain hole area vs. the probe tip. I would imagine that in icing conditions the prob tip that stick furthest into the flow would tend to cool more and thus would be more likely to freeze first.
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Old 7th Mar 2019, 16:06
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Is there any way to test the pitots for function before takeoff and perhaps during flight? Presumably pressurizing the lines and monitoring how long the pulse of pressure takes to decay would give warning if the system were obstructed.
It would obviously be a huge complication on something inherently pretty simple, but pitot problems seem to still be an accident factor.
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Old 7th Mar 2019, 16:28
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A key procedure during takeoff roll is for all pilots (PF and PM) to monitor airspeed indication which provides a very good check of pitot functionality. Takeoff is to be aborted if airspeed indicators do not come alive as expected and compare favorably between multiple sources.

Static pressure sensor viability cannot easily be checked during takeoff roll. Once airborne, the altitude readout is the best way to detect an issue with static ports. Altimeter response should be consistent with climbout and track between redundant sources. I am not a pilot so I don't know how deliberate and intentional climbout procedures tend to be with respect to confirming reasonable behavior of altimeters, but that seems prudent to me. There have been a few accidents traced to tape having been left over static ports in order to protect them during ground procedures such that the static sensors were stuck at the reading for the departure airport. With this knowledge, I would certainly keep all altitude sources in my scan during climbout if I were assigned to Row 0.
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Old 7th Mar 2019, 16:30
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I found that drawing very useful to get the answer very fast. Sorry I’m not very good at drawing. But it’s supposed to be an aircraft climbing on the left and descending on the right. I have highlighted in green the pitot and static ports. So on the left, when the aircraft is climbing: if Pitot is blocked, IAS. Will over read and if static is blocked then IAS will under read. In a descent : if Static is blocked, IAS will over read. And if Pitot is blocked, IAS will under read.

Do the drawing yourself and you will never need to think about it again. = )
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