Frozen Flight Instruments Flight Frozen | Aerosafetyworld | May 2008 | 39 Threatanalysis

ThreatAnalysis

Protection of pitot-static systems often is neglected.

BY BART J. CROTTY

n alarming number of aircraft operators placed over the static ports on the front of the fu- do not have — or do not ensure compli- selage to prevent insects or airborne debris from ance with — written policies on the use entering the system. Brightly colored streamers and care of protective covers for aircraft usually are attached to the covers/plugs to remind Apitot probes and static ports, on-site safety au- personnel to remove them before flight. dits over the past three years have discovered. Henri Pitot in the mid-1700s established the Contamination of pitot-static systems by scientific principles of measuring fluid flow pres- foreign object debris (FOD) is a safety hazard sure that later were applied to aircraft, to provide to critical flight instruments in aircraft ranging pilots with airspeed indications. Both dynamic air from Piper Cubs to Airbus A380s. A basic safety pressure and static pressure are used to derive the practice that pilots and maintenance technicians measurement so critical to flight control. Over are taught early in their primary training is to time, other flight instruments were connected to protect aircraft flight instruments from block- the basic indicated airspeed system, and pitot- age or the intrusion of any foreign material. The heating systems were incorporated to prevent the easiest way is to secure protective covers over the formation of ice that could block or reduce the pitot probe(s) and static ports, which allow neces- airflow to various flight instruments.1 sary impact air pressure and ambient pressure, re- Many aircraft accidents and incidents have spectively, to enter the pitot-static system. A few involved pitot-static systems that were affected manufacturers provide hard plastic plugs that can adversely by ice or FOD that accumulated either be inserted into the probes to protect the system. during flight or while the aircraft was parked or Many large aircraft have flush covers that can be stored on the ground, or by tape that had been Frozen Flight Instruments Flight Frozen www.flightsafety.org | AeroSafetyWorld | May 2008 | 39 ThreatAnalysis

affixed for washing or painting and subsequent- until they fail or until excessive failure or replace- ly forgotten (see “Enterprising Insects”). ment rates are documented. A complete aircraft system test and calibration check usually is not a Zero Tolerance scheduled maintenance task. Modern pitot-static and air data systems are Policies on the use of protective covers on air- designed with features intended to trap small craft pitot-static system probes and ports should amounts of moisture, dirt and other substances.2 be developed, recognizing the fact that the air- However, there is no actual standard for flight speed indicator and rate-of-climb instruments — instrument tolerance to any water or FOD — in and the air plumbing to these instruments — are other words, there is zero tolerance.3 not inspected or tested on a scheduled basis. And, although periodic tests of static pressure systems, altimeters and altitude-reporting Gaps in Protection systems are required, there is no requirement for Recent safety audits of 25 charter operators and testing airspeed indicators.4 airlines worldwide have revealed the following Some aircraft in commercial use have only problems in protecting pitot-static and air data one pilot station and only one altimeter and air- systems: The pitot cover on speed indicator. On large or transport category • No written policies; this Learjet has a aircraft, the pitot-static system is part of the air strap to secure the data system. Modern transport category aircraft • Vague written or verbal policies; angle-of-attack typically have two sources of pitot and static • Different applications of verbal policies vane; the circular pressure, and dual flight instruments, which among pilots, mechanics and ground static port, however, should reduce the impact on flight safety if one personnel; is not protected. of the pitot-static systems should become con- • No standard location in aircraft for storing taminated or blocked. But experience shows that pitot probe and static port covers; © Chris Sorensen Photography primary and alternate static systems can — • Storage locations that are not convenient and do — become to user access; completely clogged. • Pitot cover linings that are torn or frayed, Most operators burned or charred (from installation on hot have maintenance probes), or contain dirt or loose plastic; programs that call • No requirements or standards for periodic for the overhaul of examination of pitot cover linings; and, certain flight instruments — but usually • Soiled, inconspicuous or missing warning after the instruments streamers. have accumulated During more than half of the safety audits, mul- 10,000 hours or more tiple discrepancies were found. None of the audits of service since new found all the elements of a satisfactory policy for or since their last protecting pitot-static and air data systems. overhaul. Some op- Opinions among aircraft operators differ about erators maintain flight the policies and practices that should be followed, instruments under based on their experience, location, operating and an “on-condition” or environmental conditions, fleet makeup, and other “reliability” program, factors. Manufacturers’ recommendations are not which generally means always practical, and civil aviation authorities usu- that the instruments ally provide only advice and guidance, not specific continue in service requirements. The only universal agreement likely

40 | flight safety foundation | AEroSafetyWorld | May 2008 ThreatAnalysis

Enterprising Insects

recent report by the Australian and the passengers were disembarked further airspeed discrepancy. The fault Transport Safety Bureau (ATSB) with portable stairs. was suspected by the operator’s main- Ashows how quickly an unpro- “A postflight engineering inspec- tenance staff to be the result of a prior tected pitot-static system can become tion of the aircraft found what ap- pitot probe contamination migrating contaminated.1 The incident involved peared to be wasp-related debris in the to the aircraft’s air data module.” an Airbus A330 that was at the gate at PIC’s pitot probe,” the report said. After the third incident at Brisbane Brisbane Airport’s international termi- Noting that there was a “wasp on Feb. 5, the operator’s maintenance nal about 55 minutes before pushback problem” at the international gates, the manager at the airport had instructed for a flight to Singapore. report said that the March 19, 2006, line maintenance personnel to “fit pitot While rolling for takeoff, the pilot- incident was the fifth involving a “pitot probe covers as soon as possible and in-command (PIC) and the copilot, system fault during takeoff” at Brisbane remove them as close as possible to the pilot flying, noticed that the PIC’s so far that year. All the incidents departure.” A survey of the line engineers airspeed indication was 70 kt while the involved A330s, but none involved revealed different interpretations of the copilot’s airspeed indication was 110 kt. the same aircraft. Three takeoffs were instruction. “Some engineers stated that The PIC assumed control and rejected rejected; two were continued, and the pitot probe covers should be fitted if the the takeoff. The report said that the flight crews “actioned an instrument- aircraft’s turn-around time exceeded PIC’s decision to reject the takeoff was switching, non-normal procedure three hours, whereas other engineers “reasonable” and “consistent with the and cleared the fault” on their way to commented that fitment was dependent operator’s SOPs [standard operating Singapore, the report said. on the level of wasp activity present that procedures].” However, one of the aircraft sub- day,” the report said. Pitot probe covers While vacating the runway, the sequently was involved in a rejected had not been installed on the aircraft crew noticed that the wheel brake takeoff at Singapore when the crew involved in the March 19 incident. temperatures were increasing. observed an “IAS” (indicated airspeed) The report noted that the operator Although the brake-cooling fans were caution message on their electronic initiated a pest-eradication program activated, brake temperature contin- centralized aircraft monitor. “An engi- that has been successful in controlling ued to increase until the fusible plugs neering inspection at Singapore found wasp activity at Brisbane. in six of the eight wheels on the main no foreign matter in the aircraft’s pitot — Mark Lacagnina landing gear melted, causing the tires system, and the aircraft was returned Note to deflate while the aircraft was being to service,” the report said. “During the 1. ATSB report no. 200601453, “Rejected taxied on the ramp. The crew stopped subsequent takeoff, the crew again Takeoff, Brisbane Airport, Qld, 19 the aircraft and shut down the engines, rejected the takeoff as a result of a March 2006, VH‑QPB, Airbus A330‑303.” © iStockphoto

is that protection is required during long- Bart J. Crotty is a consultant on aircraft airwor- Transport Category Airplanes. Part 25.1323, “Airspeed Indicating System.” Part 25.1325, term storage and during sand, snow/ice thiness, maintenance, flight operations, safety “Static Pressure System.” Similar design or dust storms. and security, and an aviation writer based in features are required by Part 23 for non- Whatever the individual operator’s Springfield, Virginia, U.S. transport-category airplanes and by Parts experience or operational situation 27 and 29 for rotorcraft. may be, once the hazards of contami- Notes 3. Telephone interviews with Edward nated pitot-static or air data systems 1. In most aircraft, flight crewmembers must Haering, engineer and air data special- manually activate pitot or air data sensor are evaluated or re-evaluated, and a ist, U.S. National Aeronautics and Space heating systems. For more than 30 years, policy and standards are adopted or Administration Dryden Flight Research the U.S. National Transportation Safety revised, no more than one page of the Center; and Roy Gentry, vice president, Board has urged the Federal Aviation Kollsman Commercial Aviation Systems. company manual would be required to Administration to require automatic acti- establish the protection requirements vation of the heating systems (ASW, 11/07, 4. FARs Part 91, General Operating and and provide the first step in eliminating p. 10, “Automatic Heat”). Flight Rules. Part 91.411, “Altimeter or substantially reducing the problems 2. U.S. Federal Aviation Regulations System and Altitude Reporting Equipment listed above. ● (FARs) Part 25, Airworthiness Standards: Tests and Inspections.”