Welcome to AviTool’s pitot-static system FAQ Blog. Here you will find answers to esoteric, but relevant questions about testing GA and experimental aircraft pitot-static (and other avionics) systems. Pictured are the guts of our first manual pump-operated pitot-static tester. Can you identify the airspeed indicator?
FAQs are organized in the following categories:
- Symptoms of a leaky pitot or static system
- Transponder & ADS-B testing
- Pitot-Static system testing
- Checking the ELT
- Other interesting things avionics related
Symptoms of a leaky pitot or static system
Airspeed indicator reads nothing, lower than normal or is sporadic
Check the pitot tube for debris blocking the inlet.
Also check for a leak or breakage in the tubing between the pitot tube and airspeed indicator. If someone recently serviced the pitot-static system and/or instruments of the aircraft, it is possible that they flipped the pitot and static tubing entering the airspeed indicator.
Airspeed indicator is stuck on an airspeed (>0), even when in the hangar
Altimeter is reading lower than actual
Confirm that you have set the correct barometric pressure for your location. Also, you may have an “open” in the static system. Check the alternate air “switch”, a bad static drain or one left open, a leaky 
instrument case, cracked static tubing or a bad fitting connection(s). In an unpressurized aircraft these issues would account for only a few hundred feet or so error, worst case. The only way to know for sure is to perform a static leak test. To determine the accuracy of the altimeter, compare it to another altimeter where both are set to the same barometric pressure or set the altimeter Baro setting to 29.92 and compare it to the altitude displayed by the transponder.
The altimeter reads backwards when I take off
As mentioned further above, If someone recently serviced the pitot-static system of the aircraft, it is possible that they flipped the pitot and static tubing so that pitot air is feeding the altimeter and other static instruments.
The altimeter doesn’t change, reads significantly low or is sporadic
Check for a plugged static line or port. New paint, wax or a decal over the pin-sized static port will cause this problem if there is only a single static port on the aircraft.
The transponder is reporting that I’m at a different altitude than my altimeter
Transponder & ADS-B testing
When and how to test your ADS-B
What are transponder and IFR certifications?
Every 24 months, all aircraft that fly above 10,000′ MSL or in and around Class B & C airspace (larger airports), including within 30 nautical miles of a class B airport over 2500′ MSL must have an transponder and have it certified as fully functional. The tests to certify a transponder are extensive and require specialized equipment used by an FAA repair station with ratings in this area.
Aircraft that fly IFR (instruments) in the above airspace must also have their altimeter, digital encoder and static system checked. The static system cannot leak more than 100′ per minute at 1000′ over ambient (field elevation with the altimeter baro set to 29.92). Digital encoders have their baros fixed at 29.92. The aircraft altimeter cannot deviate (scale error) more than 20 feet from ambient and at numerous established altitudes up to and several thousand feet over the maximum altitude the aircraft can fly. This tolerance range expands slightly with each higher test altitude. Most non-turbocharged GA and experimental IFR aircraft are tested to 20,000′. The digital encoder, which provides altitude data to the transponder, must be accurate within +/- 125′ of the aircraft altimeter at the above mentioned test altitudes. Altimeters are also checked for hysteresis, friction and “after affect” through this range.
A note about bi-annual pitot system checks. Pitot systems are checked for leakage along with static systems as part of an IFR certification. Pitot systems that leak in excess than 10 kts over a minute when tested at 60-100 kts, are found unacceptable since excessive pitot leakage can adversely affect the accuracy of the airspeed indicator and the condition of the static system. Recall that airspeed indicators are supplied with both pitot and static air.
How accurate must my transponder be?
Short answer – within 125′ of what the aircraft altimeter reads with its baro set to 29.92.
What test equipment is used to inspect transponders and pitot-static systems? (refer to photo of equipment)
How often must I get a transponder and IFR certification?
Every 24 months. Check the aircraft logbook for the date of the last transponder and/or IFR certificate along with the acceptable altimeter error recorded during the testing process (IFR certification only).
Pitot-Static system testing
What is a pitot-static system?
A pitot-static system is actually two independent systems. The pitot system operates on a small amount of air pressure causing the airspeed indicator to function. The static system operates on a small vacuum when going up and pressure when going down. A pitot system and static system that overlap in the airspeed indicator, but their “airs” don’t blend or touch in a properly working system.
Who can perform a pitot-static leak test?
When must a pitot-static leak test be done?
What are the FAA regulations regarding pitot-static system testing?
How can I check my pitot-static system for leaks?
Is some pitot-static leakage safe and acceptable?
Why does the airspeed indicator move when performing a static leak test?
Can an aircraft be flown if a problem is found with the pitot-static system? (yes if still in cert. No, if won’t pass leak test and aircraft mechanic or avionics technician opened the system)
The airspeed indicator is actually connected to both the pitot and static ports. If you look at the back side of an airspeed indicator you will see two tubes entering it. One for either system. The pitot connection is normally located in the center and is identified by a “P”. Static air “normalizes” the airspeed so it reads properly at all altitudes since the air density changes with altitude. It doesn’t take much static (vacuum) air to move the airspeed indicator. For example, when leak testing a static system to 1000′ over field elevation (without adding pitot pressure), a connected airspeed indicator will register about 150 kts. Excessive vacuum or pressure applied to an airspeed indicator will force the pointer against its stop and may cause the pointer to slip on its shaft or deform the bellows – in either case damaging the instrument.
I give up. I can’t find the leak. What can I do now?
Seek professional help, preferably an avionics shop, as we have discovered that few A&P mechanics are well versed in pitot-static leak detection and repair. Ohio Avionics, our sister company, provides this service the Greater-Cincinnati, OH area.
Checking the ELT (Emergency Location Transmitter)
Who can check and when to check an ELT radio?
Other interesting things avionics related
How much pressure or vacuum is needed to operate aircraft instruments?
Quick answer is only a few psi. Actually, much more and you can destroy the instruments. Regarding generating a few psi of pressure or vacuum, you might wonder what your lungs are capable of. We did some tests in our pitot-static testing lab and discovered something quite interesting. Blowing, you can typically generate up to about one psi of pressure, but sucking you can generate several psi of vacuum, multiples of what you can do blowing. But, unfortunately, the mouth is not sensitive enough to detect acceptable verses unacceptable leakage in a pitot-static system. That’s where our Pitot-Static Leak Detector kit comes in.
How accurate must my altimeter be?
Can I use positive pressure and soap bubbles to find leaks in my pitot-static system?
Actually yes, but only with great care to remove ALL the instruments from the portion of the pitot-static system configuration you are testing, and then only with low positive pressure. It doesn’t take much pressure for a leak to generate bubbles.
Does the pitot-static system affect the transponder?
What does a digital encoder do?
VSI (Vertical Speed Indicator) Leaking?
Actually, yes. VSI’s are supposed to leak at a calibrated rate. We did some experiments in our pitot-static test lab on a good VSI and found the following. As you might expect, when going up and down, the leak rate is equivalent to what is being displayed on the instrument which can be as much as a few or several thousand feet per minute. So when testing a fully equipped static system that includes a VSI, you have to provide more vacuum or pressure to impact the static system than without the VSI as part of the configuration. Even more interesting, is that once the VSI returns to zero vertical speed, the VSI stops leaking or is supposed to.