Pitot-Static System Flashcards
A Pitot-Static system is, at it’s core, a pressure measuring system. Below is a basic diagram.
What three instruments are connected to the Pitot-Static System?
- Altimeter
- VSI
- Airspeed Indicator (this is the only instrument connected to both the pitot and the static system)
The pitot system only connects to one instrument. Which one is it?
The Airspeed Indicator, which is also hooked up to the Static system.
It works by reading the pressure coming in from the pitot system and comparing it to the pressure coming in from the static system
How does the Airspeed Indicator work?
The difference between the high pressure being rammed into the pitot tube, vs the low pressure blowing by the static port, gives us our airspeed.
The faster we go, the greater the difference between the two, the higher the airspeed reads
How does our angle of attack affect our Indicated Airspeed?
At a low angle of attack (straight and level flight) the oncoming air is going directly into the pitot tube. During a high angle of attack, the oncoming air is hitting the pitot tube at an angle.
This is the difference between Indicated and True Airspeed
If the static port becomes blocked, what two things can you do to create an alternate static souce if you don’t have one built into the cockpit?
1) If you can reach it (like in a C17) you can pull the cable out of the side of the plane, opening up a hole
2) In extreme cases, you could break the glass of the VSI instrument
*This doesn’t apply to pressurized AC
If you do have to turn on (or create) your alternate static source, how will this affect the instruments that rely on this source to work?
Whatever the pressure in the static system plumbing was before the port got blacked gets ‘trapped’ in the system, so:
- Altimeter - may indicate a higher than actual altitude (about 50 ft)
- VSI - will momentarily indicate a climb but then settle back down (just like during runup checks)
- Airspeed - will show a higher than normal airspeed
This is all because opening the alt static source will create a slightly lower pressure than actual*. This leads to the instruments reading a bit higher than they should.
*For whatever reason the pressure in the cockpit is just lower, we weren’t given a reason why. Of course lower pressure makes the instruments think the airplane is a bit higher/going a bit faster
What will your altimeter do if the static port becomes blocked?
It will stay stuck where it was when it got blocked
How can you tell if you static port has gotten blocked?
- Altimeter is frozen
- VSI stays on zero even in a descent or climbs
- Airspeed is not really reliable to diagnose a black static port, as it still kind of behaves as exprcted
How can you tell if your pitot tube is completely blocked (pitot tube and drain hole is blocked)
Your Airspeed Indicator will begin to behave as an altimeter.
- In a climb, your airspeed will increase
- In a descent, your airspeed will decrease
Because the pressure is ‘trapped’ in the pitot system as if was before it got blocked, as you climb the static pressure decreases and so the ASI interprets that increased difference as an increase in speed. The opposite is true for when you descend.
How can you tell if your pitot tube is blocked but the drain hole is still open?
Because there is no dynamic pressure going in the front but the pressure doesnt stay trapped in the system and slowly leaks out the drain hole, eventually the pressure inside the pitot system will be the same as outside and your ASI will slowly drop to zero. (A front blockage of the pitot tube turns it into basically a second static port)
An IFR certified airplane with have?
Pitot heat and alternate static
What will your airspeed indicator do if the static port becomes blocked?
- Under-read more and more as you climb
- Over-read more and more as you descend
If the static port line becomes disconnected inside a pressurized cabin during cruise flight, the altimeter and airspeed indicator will both read ____.
Low
