By Greg Napert
Although the pitot-static system of an aircraft is not the most complex, it is one of the most important systems on an aircraft related to navigation in helping to orient the pilot. The system is also deceivingly easy to disable or damage. A simple kink in a line, moisture that freezes, or tape left over the static ports after painting operations (this actually contributed to the crash of a transport aircraft several years ago) can result in inaccuracies in the system. And, as our navigational requirements demand more accuracy from such advances as Reduced Vertical Separation Minimums (RSVM), it becomes more important that we correctly maintain and inspect pitot-static systems.
Although virtually every maintenance facility gets involved in pitot-static system maintenance to some degree, some facilities choose to invest in equipment to accurately calibrate the pitot-static system and related instruments when necessary. One of these facilities, Banyan Air Service in Ft. Lauderdale, Florida, has gained a high level of expertise in this area — so much so, they have even developed a portable pitot-static test kit that they began to market to the rest of the industry recently.
Ken George, technician at Banyan says that the company had a large contract with a company to test 251 KingAirs to keep all of their aircraft properly calibrated. After hauling around individual instruments for a number of months, the tech staff realized they could mount the instruments into one case. This resulted in a versatile and compact test kit that they now use to perform all tests.
"It's not uncommon for static ports to be plugged with dirt, bugs, water, etc. I typically try to evacuate the line or blow it out and frequently run into instances where I have to take the lines apart and investigate further," says George. "I've also found lines that have been pinched or collapsed."
George also says that small aircraft can be more of a challenge to repair or troubleshoot than large aircraft. "Things are just more compacted into smaller spaces on light aircraft and it's harder to get access to the back of the instruments."
The basic pitot-static tests that are performed on aircraft are the same for most aircraft. However, each airplane has its own peculiarities depending on the type of instruments and whether or not it's pressurized.
George explains the basics, "Whenever you perform a calibration on the instruments that use the pitot static system, it's necessary to remove the instruments to record part and serial numbers. You then have to set up your test equipment and record the altitude you're testing to. Next, you have to take your recordings at various predetermined altitudes."
"What we are basically doing is calibrating the altimeter and the transponder. So, when air traffic control sees the aircraft and the transponder signal, and the pilot looks at their altimeter, that's the aircraft's altitude ± 125 feet. We then place a correction card with the A/C altimeter that gives the pilot a correction for altitude.
"Different tolerances are allowed in the altimeter, compared to the transponder. The tolerances also grow as you gain altitude. For instance, at 50,000 feet, you can be plus or minus almost 300 feet.
"Another thing we do is calculate the barometric scale error. If you set in different values, you check to make sure that you're plus or minus 25 feet.
"Also, we perform a vibration check. This is for aircraft that don't have vibrating encoding altimeters. This consists of manually tapping the unit to make sure the indication isn't stuck. If it has a built-in vibrator, or it is an electronic altimeter, we don't perform this test.
"We then perform a leak check at 18,000 feet. This check is for the purpose of checking the altimeter case for leaks. You simply apply vacuum to bring it to 18,000 feet and then it can't exceed a predetermined leak rate. The leak rate limit is no more than 100 feet per minute."