Pitot-static systems

Pitot-static systems

Deceivingly simple

By Greg Napert

March 1999

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."

"Finally, we perform a hysteresis check which makes sure that the bimetallic spring on the back of the needle on the indicator itself is checked at different altitudes to make sure it isn't out of tolerance."

George explains that the only difference between pressurized and non-pressurized aircraft is different leak rate values, and you test to different altitudes. "When you calibrate the entire system, (with a pressurized aircraft) you test the leak rate at 10,000 feet below the service ceiling of the aircraft and you're allowed two percent per minute for a pressurized aircraft. On an unpressurized aircraft, you are allowed quite a bit more," he says.

"These leak rates only apply to the altimeter itself. We prefer to hook up the test unit at the altimeter, since you have to take out the altimeter anyway. But, you also need to check the entire system. By knowing if the altimeter is OK, we can eliminate it as being the cause of the leak."

As a practical matter, George prefers to gain access to the instruments, bring the pitot-static test unit to the cockpit, and hook it to the instruments and the transponders. The altimeter is sending out the signal to the transponder, and the transponder broadcasts that data to the ground station, so it's important the transponder be calibrated as well. "In some cases, I hook up both the pilot's and copilot's instruments and run them up together. Then I hook everything up on the outside of the aircraft, and leak check the entire airplane — pilot and copilot side."

On some small aircraft, the pilot and copilot pitot static are combined. Larger aircraft have the entire systems separated into pilot/copilot and airdata ports. "This is important to know because not only do you have to check each of the instruments, but you have to check each of the systems as well one side at a time. If you've got a leak, you have to start isolating it based on this," says George.

There are many kits available for hooking test kits to pitot and static ports on specific makes and models of aircraft, but George sees the kits as mostly unnecessary. Every aircraft manufacturer sells its own little kit for hundreds of dollars. Manufacturer's kits typically include an adapter to fit over the pitot tube and a suction cup to fit over the static port. For an operation like ours that works on many makes and models, we would be spending thousands of dollars to have a kit for each type of aircraft. for the static port, we just take a little capillary tube, insert it in one of the holes of the static port, and seal the rest of the holes around the capillary tube. It works very well with a generic putty or clay, but be sure there's no clay or putty left in any of the static ports after you've removed it. For the pitot tube, we just obtain a piece of soft rubber tubing and slide it over the pitot tube. We can take the readings up to 50,000 feet with this arrangement with no leaks."

Graphic There's more to a pitot tube than meets the eye.

(from ASA's Avitation Maintenance Technician Series)

Ram, or impact air is taken into the front of the head of the pitot tube and directed up into the pitot pressure chamber. It is taken out of this chamber through the pitot-tube riser to prevent water from getting into the instrument lines. Any water that gets into the pitot head from flying through rain is drained overboard through drain holes in the bottom of the head, and at the back of the pressure chamber. Static air pressure is taken in through holes or slots in the bottom and sides of the head. An electrical heater in the head prevents ice from forming on the head and blocking either the static holes or pitot air inlet.

A good thing to keep in mind relative to working on the pitot-static system is the sensitivity of the equipment you're working with. In many cases, equipment such as airspeed indicators and rate of climb indicators contain small rubber diaphragm which can be blown out easily if too much pressure/vacuum is applied or applied at too fast a rate.

George says to always apply pressure and vacuum at very slow rates. "Also, you've got to be careful not to apply too much vacuum or you can literally suck the diaphragm out of the altimeter.

"You also need to be careful when arranging the equipment not to step on any of the tubing around the aircraft. This will cause a spike in the system, and again, you're trying not to damage the instruments."

Another thing to be aware of is the differences in the instruments that are connected to the system. For instance, the airspeed indicator cannot withstand as much vacuum as the altimeter, so when you apply vacuum to the static ports, you have to apply vacuum to the pitot tube in order to equalize the pressure in the airspeed indicator. "Just remember that the airspeed indicator is also connected to the static port and you need to protect it from any pressure or vacuum you apply to the static port while testing other instruments," says George.

"You can run the airspeed indicator up by itself from the pitot tube, but you've got to do it very slowly and be sure not to go past the limit of the airspeed indicator or you will rip the bellows."

As with any system on an aircraft, you need to make a complete and thorough study of each and every aircraft before beginning your work to understand all the potential consequences of your actions. The pitot-static system may appear basic, but the instrumentation attached to it is sensitive and highly accurate equipment, which you need to have a working knowledge of before applying any pressure or vacuum to the system.

According to George, Banyan trains all of their technicians who work on these systems to review aircraft maintenance manuals, study the regulatory requirements associated with pitot-static testing, and to review all equipment installed on an aircraft prior to working on that aircraft. As well, they require the technicians to use a customized checklist designed for the equipment they are using to test the aircraft, and for the type of aircraft they are testing.