Partially blocked static and pitot lines? What happens?

[SuperTed]

I know what happens when the static and pitot lines become totally blocked. However, what happens to the pressure instruments when the pitot and static lines become partially blocked.

Many thanks,

ST

[Checkboard]

A partailly blocked pitot tube won't transmit the full dynamic pressure to the ASI and/or Air Data computer, causing the airspeed to underread.

A partially blocked static port is a little different as the static port is dependant on the airflow around the opening, so a partial blockage may result in instrument lag, or erroneous readings, depending on the nature of the blockage.

[Keith.Williams.]

Checkboard,

I think we need to be a bit careful here.

If either of the pipes become partly blocked it will reduce the rate at which changes in pressure are detected by the instruments and ADC. But this does not mean that the full pressurs will never be detected.

If for example the pitot pipe becomes blocked, the ASI will be slow in detecting speed changes, but will give an accurate reading after a brief period in any given flight condition.

The effect will be similar if the static pipe becomes partly blocked. You will eventually get accurate indications of speed and altitude but the repsonse rates will be lower. Indications of rate of change will also be incorrect.

So a partly blocked pitot pipe will give:

Under indication of speed when accelerating or descending.
Over indication of speed when decelerating or climbing.
Correct indication of speed in constant speed level flight

And a partly blocked static pipe will give:

Over idication of speed and altitude when climbing.
Under indication of speed and altitude when descending.
Under indication of rates of climb and descent.
Correct indications of speed and altitude in constant speed level flight.


I suspect there are some other but cannot think of them at the moment!!!

[EchoTango]

Keith Williams

The bit you missed was the static line partially blocked, with the blockage affecting the airflow over the vent. Hornets do it with dobs of mud.. .Then you get unpredictable static pressure.

ET

[john_tullamarine]

As an aside and following on from EchoTango ..

In addition to the previous commentaries, consider that

(a) a significant concern with pitot static systems (which have already been through the certification process) relates to the consequences of disturbed flow (ie different to the calibration conditions) in the vicinity of the static source.

(b)a very significant part of the certification flight test process involves the determination of PEC data.

Such airflow problems can arise from ill-considered or tested modifications or other physical changes upstream of the sensing location including comparatively minor local damage in the vicinity of the static source.

It is very easy to generate quite significant instrument errors from these sorts of problems.

[domestic]

A real life experience from a partially blocked pitot in a 737 classic. Takeoff to 80 kts normal then a big lag, then a constant error of about 20kts throught the whole flight. The lag dissapeared for landing but the error remained constant, the villian a well cooked and rather large bug stuck in the front of the pitot tube.

[Keith.Williams.]

EchoTango and domestic,

I think both of your examles illustrate a slightly different problem. That is one where debris disturbs the airflow, rather than simply partly blocking the tubes. If debris creates turbulence over the inlets, the true pitot or static pressure will not be felt in the tubes. The fact that domestic's error was greatest at low speed where pich attitude would be greatest, seems to support this explanation. SuperTed is studying for his ATPL exams, so I suspect his original question was related to this. I therefore dealt with the results that I have observed in past feedback questions.

. .SuperTed,

You might like to try these additional questions.

1.. .If the pitot pipe becomes partly blocked?

a. The IAS and ROC will be too low when climbing.. .b. The IAS and ROC will be too low when descending.. .c. The IAS and ROC will be too low at all times.. .d. The IAS will be too low when descending and too high when climbing.

2.. .If the pitot pipe becomes partly blocked?

a. The IAS will be too high when descending.. .b. The IAS will be too high when accelerating.. .c. The IAS will be too low at all times.. .d. The IAS will be too low when accelerating.

3.. .If the static pipe becomes partly blocked?

a. The IAS and ROC will be too low when climbing.. .b. The IAS and ROC will be too low when descending.. .c. The IAS and ROC will be too low at all times.. .d. The IAS will be too low when descending and too high when climbing.

4.. .If the static pipe becomes partly blocked?

a. The IAS will be too high when descending at constant IAS.. .b. The IAS will be too high when accelerating at constant altitude.. .c. The IAS will be too low at all times.. .d. The IAS will be too low when accelerating at constant altitude.

5.. .If the pitot pipe becomes partly blocked?

a. The ASI will over read and the error will be greater when climbing at constant CAS than when climbing at constant TAS.. .b. The ASI will over read and the error will be greater when climbing at constant TAS than when climbing at constant CAS.. .c. The ASI will under read and the error will be greater when climbing at constant CAS than when climbing at constant TAS.. .d. The ASI will under read and the error will be greater when climbing at constant TAS then when climbing at constant CAS.

. .My answers are:

1. d. Because the ROC indicator (VSI) does not use pitot pressure.. .2. d.. .3. a.. .4. a.. .5. b. Because the rate of change of pitot pressure is greater when climbing at . . constant TAS, than when climbing at constant CAS.

[Checkboard]

Ahhh, the difference in answering a question about flying and answering an exam question!

[EchoTango]

Keith Williams

You are quite right. I was simply nitpicking a very thorough answer.

ET

[SuperTed]

Can I just thank everyone who has answered my thread. Thankyou especially to Keith!

[PWR+ATT=PERF]

Partially blocked static vent from actual experience . .1/ Take off roll - normal indications

2/ Climb - airspeed steadily reducing with height gained, past stall, past zero, to minus 140 kts. VSI close to zero, Altimeter indicating the altitude corresponding the height of the blockage (with a partial blockage it very slowly climbs at a lesser rate than the aircraft.

3/ Cruise - Everything slowly comes back to normal as the pressure equalises to the correct outside pressure.

4/ Descent - The exact opposite to climb, the airspeed indication increased past VNE and right on past zero again. In the circuit 180 kts on final, touched down 140 kts and turned into the taxiway indicating 80 kts (Piper Comanche)

2nd occasion right hand seat in PA31 Navajo IF take off "dont lower the nose any further you have a blocked static vent, hit your alternate static source)

This I teach my IFR students. .If you pre think this scenerio prior to an IFR takeoff, and can immediatly put your hand (eyes closed for practise) on the alternate static source, then you should live to tell the tale.

[GE 90]

Personal experience of one partialy blocked pitot probe. Disagreement in Capt / F/o's instruments at V1 and aborted Take Off. 10 wheel and brake changes, 5 slide changes, 10 hours of investigation to find bug had flown away. . .Not to mention trouble of collecting pax from end of African (snakes and all) runway and hotac.. .Still, made a change from sunny LGW. <img src="wink.gif" border="0">