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 has largely fallen upon the shoulders of the 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 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 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 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 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 , 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 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. d i fferential pressure in one minute. instructions on the performance of the Fluorescent streamers are attached to Why the difference? In an unpressur- tests in the aircraft. If you are an inde- wherever tape is applied to block a ized aircraft, the static lines could leak pendent avionics shop and do not have static port. The Advisory Circular rec- into the cabin without much influence access to a complete airframe library, ommends that the connections be on the altimeter reading. The cabin ask for the instructions from the air- made directly to the static ports if pos- pressure may differ from outside ambi- craft operator or the aircraft manufac- sible. It does allow for the connection ent pressure by 100 feet or so. In pres- turer. to a static system drain or tee connec- surized aircraft, a static leak in the A practice that I have found helpful tion with the static ports sealed off if cabin may cause huge errors in the is to compile a book of pitot-static test direct connection to the ports is static instruments if the internal cabin information. This book gathers togeth- impractical. Such an alternate connec- pressure is much higher than the out- er all of the information that is typical- tion must be made where the system side ambient pressure. In aircraft ly collected as you test various aircraft. integrity can easily be checked after capable of nine inches or more of dif- Bear in mind that a book such as this the test is completed and the system is ferential pressure, the equivalent dif- must be considered “For Reference returned to normal. ference in altitude can exceed 25,000 Only” and the information contained With the test set sitting on a cockpit feet. therein should be supplanted by cur- seat, it is a necessity to proceed cau- Since the aircraft pressurization sys- rent maintenance information from an tiously. If you begin introducing vacu- tem maximum differential is expressed FAA approved source. In my book, um and do not see the immediate in PSI, it is necessary to either connect we have printed out all of the relevant results you expect, STOP! Recheck an accurate vacuum gage referenced to FARs, Advisory Circulars, ADs, etc. your setup. Once you are sure every- ambient and connected to the static that pertain to pitot-static, transponder thing is fine and you see the appropri- system under test or use the aircraft or and altimetry equipment. In addition, ate EFIS response or altimeter needle test altimeter as an equivalent. this three ring binder has a tab for each movement, it is OK to move ahead. Obviously if you use a vacuum gage, aircraft type that we test and behind It is desirable to keep a little bit of when it reads the maximum cabin dif- these tabs are the piping diagrams and airspeed showing on the airspeed indi- ferential pressure, you have reached latest airframe test procedures. We cator while running a pitot-static sys- the proper altitude to perform the leak also keep the latest information on the tem “up.” This assures me that the air- test as required by the regulations.

42 AVIONICS NEWS • APRIL 2004 Some of the electronic test standards ders of the avionics technician with such as the Setra M370, can be set to I certify that the altimeter and static RVSM testing responsibility to verify display in a variety of units including system tests required by FAR Par t that the required equipment for RVSM inches of mercury. When doing a 91.411 have been performed. operations is still in the aircraft and pressurized aircraft leak test with the operating satisfactorily. This means Setra, change the display to read in The Altimeter has been tested to that an equipment inventory, inches of mercury, reference the dis- ______feet on ______testing, alerter testing and detailed play to zero at the ambient pressure, (date altimeter tested). inspections of the critical areas around and evacuate the system until you static ports will be a part of the RVSM reach the desired differential pressure. Signature______maintenance program in addition to At that point you can perform the leak the altimeter/air data tests themselves. test. Date ______FAR 43 Appendix E requires that In order to use the aircraft or test (of Static system test) the date of the test and the altitude to altimeter as a vacuum gage, it is nec- which the test was performed be writ- essary to do the following: Certificate No.______ten on each altimeter (air data comput- 1. Convert the local barometric er). This is typically done with sticky- pressure to PSI using the formula back labels. In the case of air data PSI=Inches of Hg/2.036. The baro- The following sign-off is more spe- systems, I affix a label to both the metric pressure can be obtained from cific and complete, lending itself to remote computer and the altimeter the altimeter by setting the altimeter to partial testing (non-RVSM aircraft instrument in the panel. zero feet and reading the baro scale of only) of the onboard altimetry or stat- It is also a common practice to fill the instrument. ic systems: out altimeter correction cards and affix 2. Subtract the maximum cabin dif- them to the logbook entry or give them ferential pressure in PSI from the I certify that the altimeter and static to the aircraft operator. Remember to value obtained in Step 1. system tests required by FAR 91.411 reverse the sign of the error when 3. Convert the PSI value obtained in have been performed in accordance transposing the corrections on the card Step 2 to inches of mercury using the with FAR 43, Appendix E (para. Noted) from the altimeter test sheet. This is formula Inches of Hg = PSI x 2.036; necessary if the altimeter test sheet 4 . Convert the test pressure ex- 1. Static System per Para.(a) asks for the instrument error refer- pressed in inches of mercury to the test write in left,right, or left and right enced to the test device. For example, altitude by using a conversion chart if the master altitude is 1,000 feet and based upon the document U.S. 2. L. Altimeter per Para. (b) to the altimeter under test reads 985 feet, Standard Atmosphere, 1976. ______,000 Ft. S/N______(if req’d) then the instrument error is entered as There are no specific guidelines in on date______. –15 feet. If the correction card states the FARs for pitot systems. When to add the correction algebraically there is also no data in the airframe 3. R. Altimeter per Para.(b) to when the altimeter reads X, then in maintenance manual for testing the ______,000 Ft. S/N______(if req’d) this example it will be necessary for pitot system, a good rule of thumb is to on date______. the pilot to add a +15 feet to an altime- test at 200 knots or 230 mph, or red- ter reading of 1,000 feet in order to line, whichever is lower. The pitot 4. Integrated System per Para.(c) arrive at the corrected altitude. system should not leak more than 2 Item No. ______list #s above Confused? Luckily I have rarely seen knots or 2 mph in one minute. Tested by______a pilot using an altimeter correction The job is not complete until the Date______card during my long career. paperwork is done. With the inspec- In summary, all I can say to you tions required by FAR 91.411 the sign- young, new avionics technicians with o ff is especially important. T h e In addition to the sign-off above, an altimeter testing responsibility is that Maintenance Record entry example RVSM equipped aircraft would have it will be a learning experience for all listed in AC 43-203B is a good start an entry referencing the specific of us as the deadline for domestic U.S. but is too vague for aircraft equipped RVSM test procedure that was per- RVSM approaches. Who says you with dual systems: formed. can’t teach an old dog new tricks? It will probably fall upon the shoul- Ruff, ruff! ❑

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