"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."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.
Pitot-Static Systems Basic for flight By Jim Sparks March 2000 Air is one of the basics needed to support life. In fact for most of us, it is also the basis for our...
Every Two Years FAR 91.411 and 91.413 testing requirements By Jim Sparks May/June 2001 It comes with the same surefootedness as the tax collector, but with a somewhat reduced...
Aircraft Instruments Knowing how to handle is as critical as how to repair By Ron Rucker September 1999 Instruments have, for many years, been the stepchild of avionics. Although...