AIR SPEED KNOTS 40 60 80 100 120 140 160 180 200 220 240 10-84 Figure 10-138.  An analog pitot-static system test unit (left) and a digital pitot static test unit (right). Figure 10-137.  An airspeed indicator makes extensive use of range  markings.  Pitot-Static System Maintenance and Tests system to remove any moisture during maintenance. Lacking  this, dry compressed air or nitrogen may be blown through  the lines of the system. Always disconnect all pitot-static  instruments before doing so and always blow from the  instrument end of the system towards the pitot and static  ports. This procedure must be followed by a leak check  described below. Systems with drains can be drained without  requiring a leak check. Upon completion, the technician  must ensure that the drains are closed and made secure in  accordance with approved maintenance procedures.  Aircraft pitot-static systems must be tested for leaks after  the installation of any component parts or when system  malfunction is suspected. It must also be tested every 24  months if on an IFR certified aircraft intended to be flown  as such as called out in 14 CFR section 91.411. Certified  airframe and A&P technicians may perform this test.  The method of leak testing depends on the type of aircraft,  its pitot-static system, and the testing equipment available.  [Figure 10-138]  Essentially, a testing device is connected  into the static system at the static vent end, and pressure is  reduced in the system by the amount required to indicate  1,000 feet on the altimeter. Then, the system is sealed and  observed for 1 minute. A loss of altitude of more than 100  feet is not permissible. If a leak exists, a systematic check of  portions of the system is conducted until the leak is isolated.  Most leaks occur at fittings. The pitot portion of the pitot- static system is checked in a similar fashion. Follow the  manufacturer’s instructions when performing all pitot-static  system checks. Title 14 of the Code of Federal Regulations (14 CFR) part  91, section 91.411, requires this test, as well as tests on the  pitot static system and on the automatic pressure altitude  reporting system. The certified airframe or A&P mechanic is  not qualified to perform the altimeter inspections. They must  be conducted by either the manufacturer or a certified repair  station. 14 CFR part 43, Appendix E detail the requirements  for these tests. Water trapped in a pitot static system may cause inaccurate or  intermittent indications on the pitot-static flight instruments.  This is especially a problem if the water freezes in flight.  Many systems are fitted with drains at the low points in the  In all cases, pressure and suction must be applied and  released slowly to avoid damage to the aircraft instruments.  Pitot-static system leak check units usually have their own  built-in altimeters. This allows a functional cross-check of 