Airspeed gauge showing zero

[Killer Instinct]

Hey everybody!  I was hoping I wasn't the only one with this problem.  Lately when I have been flying I have been going along quite nicely when my airspeed gauge will suddenly only show zero.  The plane is still bombing along but I am showing no airspeed.  At this point I have to save the flight and exit and then go back in.  I sometimes have to do this three to five times per flight.  Any ideas?  I am a bit stumped here!  Thanks

Mark

[BAW0343]

Turn on your pitot heat  ;D  The little tube that collects this data can get frozen over in cold weather showing an airspeed of 0

[macca22au]

Nothing on the gauges but the maker's name.  Time to bale out!

[Wingo]

Actually, BAW, if you get pitot tube icing it will freeze the ASI at the speed at which the blockage occurred. It wont change with acceleration/deceleration but will indicate an increase in speed as you climb and decrease in speed as you descend.

[Brett_Henderson]

Actually, BAW, if you get pitot tube icing it will freeze the ASI at the speed at which the blockage occurred. It wont change with acceleration/deceleration but will indicate an increase in speed as you climb and decrease in speed as you descend.

Unfortunately, MSFS does not replicate that scenario. Once the tube freezes, the IAS goes to zero.

But even in R/L, the tube AND its drain need to ice over for that VERY perilous event.

If you're cruising along at 110kias and the tube/drain ice up (Locking the 110kias pressure in the pitot system).. you might not even know it, because all the ASI knows is the DIFFERENCE between what the tube sees and what the static port sees... And if you're plugging away on instruments and on your scan, you see an IAS decrease, your first instinct is to pitch down .. and if that IAS decrease is from a frozen over tube, you're ALREADY descending.. and when pitching down FURTHER decreases IAS... you can get confused and flustered..

scary stuff

[Mr.Mike]

Brett has done his home work

BLOCKED PITOT SYSTEM

Taken from FAA-H-8083-25 "Pliot's Handbook of Aeronautical Knowledge"

Page 6-8
"The pitot system can become blocked completely or
only partially if the pitot tube drain hole remains open.
If the pitot tube becomes blocked and its associated
drain hole remains clear, ram air no longer is able to
enter the pitot system. Air already in the system will
vent through the drain hole, and the remaining pressure
will drop to ambient (outside) air pressure. Under these
circumstances, the airspeed indicator reading decreases
to zero, because the airspeed indicator senses no
difference between ram and static air pressure. The
airspeed indicator acts as if the airplane were stationary
on the ramp. The apparent loss of airspeed is not
usually instantaneous. Instead, the airspeed will drop
toward zero. [Figure 6-8]

If the pitot tube, drain hole, and static system all
become blocked in flight, changes in airspeed will not
be indicated, due to the trapped pressures. However, if
the static system remains clear, the airspeed indicator
acts as an altimeter. An apparent increase in the ram air
pressure relative to static pressure occurs as altitude
increases above the level where the pitot tube and drain
hole became blocked. This pressure differential causes
the airspeed indicator to show an increase in speed. A
decrease in indicated airspeed occurs as the airplane
descends below the altitude at which the pitot system
became blocked. [Figure 6-9]

The pitot tube may become blocked during flight
through visible moisture. Some airplanes may be
equipped with pitot heat for flight in visible moisture.
Consult the AFM or POH for specific procedures
regarding the use of pitot heat."


Hope this clears it all up

[Brett_Henderson]

It's one of the most important scenarios in which you'll be drilled, during your instrument rating oral exam, and really makes you realize how important the complete scan, is... and how important it is to differentiate between accurate and erroneous data, when your instruments conflict.

Now ponder the opposite... (this makes my hair stand on end).. Your tube and its drain ice over while you're flying IMC and can barely see your wing-tips. Then, while tending to the radios, or some other distraction, you look back to see your airspeed increasing. Your first instinct is that you're descending, so you pitch up a bit. This of course makes the ASI show a further increase (absolutley contrary to what you'd expect). Now.. a quick glance at your attitude-indicater and altimeter can confirm that you are climbing and that the ONLY way for the indicated airspeed to increase is from a frozen tube, but we aren't always quite so calm and rational (especially as a low-time instrument pilot, alone in the cockpit). Anyway, back to worst-case scenario...

[Wingo]

Please excuse me, I have to clean the mess off the seat after that story.  :o

I have never been taught about the air bleed in a pitot system. The only two possibilities are that they aren't common on small GA aircraft and common on larger, more sophisticate aircraft, or, my teachers have been a bit lax. I think it might be the former rather than the latter.

[Brett_Henderson]

I'm only intimately familiar with light, piston singles and a few twins, and it's common on all that I've flown. If you fly a C172, it's the little pin hole on the bottom rear of the pitot tube.

You wouldn't be expected to understand that scenario, until you're taking instrument training. I think that a thorough understanding of the pitot/static system is not drilled as it should be, even for simple PPL training.

Throw your instructer a curve and ask him what happens when the tube AND its drain ice over  :)