Pitot-Static Attack
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T E C H N I C A L PITPITOTOT-ST-STAATICTIC AATTTTAACKCK BY BRAD PRIMM he task of keeping the air-data accurate on-board general avia- Ttion aircraft has largely fallen upon the shoulders of the avionics shop. It is a duty that cannot be taken lightly. Every two years, or even more frequently for RVSM equipped air- planes, the aircraft pitot-static systems are assaulted with a variety of test equipment, needles, fixtures, hoses and vacuum/pressure pumps. The fol- lowing discussion is not going to include detailed instructions for test- ing the altimeters or air data comput- New RVSM tester on left, old reliable on the right. ers themselves but rather this will be a general overview of SAFE altimeter them. Routing the hoses under the air- The requirement to test altimetry in system testing in the aircraft. craft belly as much as practicable will general aviation aircraft stems from One of my biggest fears as a prac- help prevent all but the shortest the FAR 91.411 regulations. In that ticing avionics technician with altime- midgets from tripping. regulation, an owner/operator of an ter testing responsibility has been the Another one of my recurring night- aircraft is required to test the altime- fear of someone tripping over a hose mares has been the fear of not remov- ters, static systems, air data comput- while an altimeter system test was ing all of the tape and tape residue ers, and integrated pressure altitude underway. You need to be neat in used to seal off alternate or duplicate reporting system every two years if the your setup and not leave hoses in static ports and drains after the testing aircraft is to operate under instrument walkways or other areas that might is completed. I kept florescent colored flight rules (IFR). There are no rules invite someone to step on or trip over tape streamers in the pitot-static hose in the FARs specifically requiring the kit box and hung them from each port periodic check of the pitot systems or that was sealed off. The streamers airspeed indicators unless of course, made sure that I did not forget to the aircraft is equipped with an air- remove the tape from any port. data computer. Any periodic require- Another good practice is to make a ments for testing pitot lines and air- separate item on the work order that speed indicators stem from the mainte- says, “Remove all tape, tape residue nance manual inspection requirements and/or test equipment from the aircraft of the airframe manufacturer. The after Pitot-Static tests are completed.” requirement for RVSM testing is This item will then get a separate sign- going to come from FAR 91.180 and off and prompt the inspector to do that Appendix G of that same Part. The last walk-around to ensure all is back Conventional static ports, affectionately shorter testing intervals and tighter tol- known as Pepper Ports. to normal. erances of RVSM aircraft stem from 40 AVIONICS NEWS • APRIL 2004 the approved maintenance programs experience must prevail to ensure that developed for the RVSM approved a cabin pressure gage or hidden air- aircraft. speed sensor is not damaged or back- There are two primary ways to per- pressured while performing the checks form altimeter checks. In the past, it required. was normal to remove the altimeters Advisory Circular 43-203B should and send them to an instrument shop. be required reading for any technician After receipt of the certified altimeters performing in-aircraft static tests. The from the instrument shop, the system following excerpt from paragraph 9.a. was leak checked and the inspection hits the nail on the head: “Before any was signed off. In recent years, the static system is tested, it should be testing of the altimeters and encoders determined that the design limits of while still installed in the aircraft has instruments attached to it will not be become the norm. This can be tricky exceeded during test. To determine since many older aircraft come from this, it is necessary to locate and iden- the days when this was not the com- tify all instruments attached to the sys- mon practice and there may not be tem. In addition to the altimeter, air- instructions in the maintenance manu- speed, and rate of climb, many air- al for the step-by-step performance of planes use static pressure for the oper- these checks. Common sense and ation of autopilots, flight recorders, air data computers, altitude reporting dig- itizers, etc. The use of a static system The static hose kit suitcase provides a convenient way to wheel out the tester for diagram of the airplane involved may the attack. be helpful (definitely!) in locating all of the instruments. If a diagram is not will still be at ambient pressure. available, the instruments can be Normally, I always try to run both located by tracing the physical instal- sides up together because then it does- lation.” Even though this advisory cir- n’t matter if such a cross connection cular was written in 1979, it is more exists. relevant now than ever before. Since Advisory Circular 43-203B Section 9.b. of the A d v i s o r y was written in 1979, it does not con- Circular talks about the damage that tain any guidance for the certification can be done to instruments that are of RVSM equipped aircraft. The best connected to both the pitot and static place to find general FAAguidance on Both pitot tubes contain two static ports. Another pitot tube with dual static ports is on systems when only one system is RVSM maintenance requirements is the pilot’s side as well. evacuated. This can cause the differ- w w w 1 . f a a . g o v / a t s / a t o / r v s m 1 . h t m . ential pressure design limits of such The website has the latest FAQ’s and instruments to be exceeded. As the FAAorders referencing RVSM includ- Circular says, if you tie both the pitot ing document 91-RVSM, Interim and static systems together, then you Guidance Material on the Approval of will have zero differential pressure Operators/Aircraft for RV S M regardless of the amount of static sys- Operations. This document refers to tem evacuation. In complex aircraft generalities of design and defers all with dual pitot and static systems, testing details to the approved mainte- there is always the worry that an after- nance program for the RV S M market air data device may be con- approved aircraft. nected to the pilot’s pitot and the copi- More and more of these approved lot’s static. In this case, you would maintenance programs are now requir- backpressure or exceed the device lim- ing that aircraft testing be performed its even if you tie the pilot’s pitot and with all pitot tubes and static ports tied Close up of combined pitot-static tube static together since the copilot’s side Continued on following page AVIONICS NEWS • APRIL 2004 41 PITOT-STATIC various Air Data testing procedures speed is not back-pressured. This is Continued from page 41 such as Honeywell’s Te c h n i c a l typically done by closing the crossover together during test. In fact, many newsletter 23-1980-04 and Collins’ valve slowly while the altimeter is RVSM test kits come with extra hoses Service Information Letters such as climbing until some airspeed is indi- instead of blanking adaptors so all of ADC-86( )/850( ). The book also has cated. On fancier test sets such as the the pitot-statics can be tested together. all of the related Repair Station forms DSP-400, you select the desired air- Dassault and Airbus are two manufac- for recording the results of the testing. speed and the test box controls the turers that are now writing their pro- In addition to the book, we keep all crossover automatically. W h e n grams with this as a requirement. I of our hoses, AN fittings, putty, fluo- descending, always use the pitot vent, suspect the reason for this is to more rescent streamers, tape, etc. in a fiber- never the vacuum vent to bring the closely simulate in-flight conditions glass suitcase that is equipped with system “down.” This will automatical- and to allow a comparison between wheels and a telescoping handle. This ly keep some airspeed present in the both altimeter systems. Another trend makes it easy to wheel out the Barfield system. in the RVSM area is to forbid the use test set together with the hose case. Two different standards are applied of needles, putty and stretchy hoses Remember, time is money and any- to testing static systems. If an aircraft over the pitot tubes. I am sure the fear thing to make the operation more effi- is unpressurized, FAR 23.1325 states is that residue or contamination from cient will be appreciated by your cus- that the static system must not leak these previously accepted practices tomers. more than 100 feet in one minute after could contribute to RVSM altimetry Once out at the aircraft, the setup being evacuated to 1,000 feet above error budgets. begins. After reviewing testing infor- ambient. In pressurized aircraft, the Whenever possible, follow the mation and piping diagrams, the FAR states that the system must not maintenance instructions listed in the appropriate test adapters are fitted to lose more than 2 percent of the equiv- airframe manual. Most newer aircraft the aircraft, opposite ports are taped alent altitude of the maximum cabin maintenance manuals have detailed o ff and the hoses are connected.