July 1, 2004
Steven W. Ells
Reaching out to point her finger, the kindergartner asked, "Mom, what's that?" Her mother answered that it was a typewriter, and that almost everyone used to have one.
With the recent arrival of glass panels in piston-powered airplanes, children might one day ask the same question when they first glimpse an analog airspeed indicator. We can't foresee the future, but for now the instruments of the pitot-static system — the airspeed indicator (ASI), vertical speed indicator (VSI), and altimeter — are still a vital part of every airplane's instrumentation. In fact, regulations require even the most basic airplane to have an airspeed indicator and altimeter for day VFR flight.
When turbine-powered airplanes started to expand the performance envelope, the probes, ports, plumbing, and instruments of the pitot-static system were augmented by air-data computers and mach meters. Similar devices, which at their most basic crunch the raw data by factoring in temperature, are beginning to show up in general aviation airplanes. But most of us still use analog instruments to tell us how fast we're going; whether we're climbing, level, or descending; and how high we're flying.
This trio of air-pressure instruments is extremely reliable. And while variables, such as pitot-tube installation error and deviations from standard day conditions, cause the indicated airspeed, altitude, and rate of climb to be close approximations, the reliability and low cost of these systems offset these small errors.
A basic pitot system starts with a tube (called a pitot tube) that projects forward and is aligned parallel to the airplane's direction of flight. An opening in the front of the tube connects to a continuous hose that runs to the airspeed meter. Pitot pressure is always above zero when an airplane is moving.
A basic static system consists of openings called ports that are located perpendicular to the direction of flight. The static ports on single- and twin-engine piston-powered airplanes are most often located on the sides of the fuselage, although another common location is in the side of the pitot tube.
There are two ports, one on each side of the airplane or pitot mast since there are times when the pilot either chooses to slip or skid the airplane, or when meteorological conditions such as turbulence cause the airplane to slip or skid in forward flight. Multiple ports maintain the needed reference to static pressure. The static air is piped through rigid and flexible hoses to the airspeed indicator, vertical speed indicator, and altimeter. The static system is also used to supply static air for the altitude-reporting portion of the transponder.
Heaters are installed on the pitot tubes and may be installed on the static ports of airplanes that are flown into clouds and in icing conditions. Airplanes with flight-into-known-icing certification are equipped with high-capacity heaters.
Although an alternate static source isn't required by regulation to fly in IFR conditions, this pilot-controlled valve provides static air if the static ports become clogged by ice or debris. The alternate static source opens the static system to cabin air.
Pilots are taught that the venturi-like conditions of the relative wind flowing over the airplane cabin cause cabin static air pressures to be lower than static port pressures, so the following is usually true to some degree — the altitude indicated is higher, the airspeed is higher, and the VSI indicates a climb when the airplane is in level flight.
Alternate static source pressure is also affected by ram air coming into the cabin through the fresh-air ventilation or cabin heat systems.
The inner workings of the three instruments of the pitot and static systems are invisible to the pilot, but they're fascinating to consider.
The airspeed indicator, vertical speed indicator, and altimeter rely on one or more aneroid wafers constructed of thin metal, most commonly brass, to measure the very small changes in air pressures in the pitot and static systems. Each pitot-static instrument case is sealed — changes in pressure within the pitot and static systems are so small that the aneroid wafers (hereafter called diaphragms) must be isolated from cabin air-pressure changes. The diaphragms are flexible and since instrument internal friction has to be kept to a minimum, all bearings are dry, with tiny shafts riding in Lifesavers-shape jeweled bearings. The pressures these instruments measure are very small. The impact pressure at 40 mph is 0.028 pounds per square inch (psi) while at 160 mph it only increases by 0.434 psi to 0.462 psi.
Each instrument case is connected to the static system through a screw-on or push-on hose fitting that is located on the back of the instrument case. The airspeed indicator has a second fitting for the pitot pressure.
Impact air from the pitot tube enters the diaphragm of the ASI, which causes it to expand since the impact pressure is higher than the static pressure surrounding the diaphragm. This expansion is translated, through mechanisms with lyrical names such as the hairspring, handstaff pinion, rocking shaft, and sector gear, to movement of the airspeed needle.
The VSI measures the difference between the static pressure inside its diaphragm and the static pressure surrounding the diaphragm. If there's no difference between the two pressures, the needle points at zero, meaning the airplane is neither descending nor climbing. Both the air in the case and the air inside the diaphragm are connected to the static system, but the air in the case is connected to the system through a small hole. It's small on purpose. It's called a restricted orifice, and it prevents static air from moving into or out of the case at the same rate as it moves into or out of the diaphragm. Climbing or descending creates a pressure imbalance between the static pressure inside the diaphragm and the static pressure outside the diaphragm. Any imbalance moves the needle.
The altimeter also uses a diaphragm; in fact, it uses a series of diaphragms. After all, the range of indication is far greater in an altimeter than in the other two pitot-static instruments. Some pressure altimeters are certified for an operating range of 50,000 feet. Unlike the other instruments in the pitot-static system trio, one end of the diaphragm is rigidly attached to the case. Changes in the static pressure are balanced against a stable pressure (a vacuum) inside the bellows so that when the static pressure changes, the overall dimension of the bellows changes, and this is translated into altimeter needle movement.
Altimeters have one more significant difference — the pilot is able to compensate for variances from standard day pressures (pressure altitude) when he adjusts the number in the Kollsman window before takeoff and periodically during flight. Although the VSI has a faceplate-accessible compensation adjustment, pilots are not permitted to make any adjustments.
Aerodynamics for Naval Aviators, a basic text that uses math and physics to explore the aerodynamic forces on airplanes, says "An aircraft altimeter is a sensitive barometer calibrated to indicate altitude in a standard atmosphere."
Pitot static instruments haven't changed a great deal since the 1940s and 1950s. There have been improvements in manufacturing techniques and materials — diaphragms were originally soldered together; now the halves are joined by electron-beam welding in a vacuum chamber.
According to Frank Leoni and his cohort Chris Zigler at Air Instrument Repair at General William J. Fox Airfield in Lancaster, California, "The pitot and static system instruments built after the war [World War II] were built right. They have jeweled movements and bulletproof cases."
Leoni warned that some early instrument dials were coated with radium or phosphorous to increase the visibility of the dial in low-light conditions. The radium dials must be removed from service when the instrument is sent in for repair or overhaul. There is no published time between overhauls (TBO) for pitot-static system instruments. The most common cause of inaccurate readings is corrosion caused by moisture, dirt, and dust.
To lessen the impact of moisture, airframe manufacturers designed the pitot system plumbing to ascend from the static ports before routing it forward or else installed an easily drainable low point, or moisture trap, in the static system. The cap on the low point should be removed at each annual, or as necessary.
Instrument needle lag or needle jerkiness is the best indicator that it's time to send an instrument to a certified shop for repair or overhaul. Wear and friction show up during the hysteresis or elastic error tests that are part of the altimeter and ASI recertification. Hysteresis means to lag behind and signifies that the instrument has lost its ability to adapt to rapid change.
Pilots can check the altimeter calibration prior to every flight. Simply set the reading in the altimeter Kollsman window to the local barometer setting. If the altitude pointer is off the field elevation by more than 75 feet, the altimeter needs to be recalibrated.
An ASI that does not return to zero is also sending the pilot a message that it would like a visit to the local instrument repair shop. Airspeed indicators are also tested for calibration during overhaul. The standard is plus or minus 2 knots for a zero-to-150-knot-range instrument.
FAR 91.411 requires that the static systems of airplanes flown under IFR be checked for leakage in accordance with FAR 23.1325 every two years. In addition, each altimeter must be checked for compliance with the parameters in FAR Part 43, Appendix E, every two years. If an altimeter is in good shape, these tests can be done in the airplane by a properly rated shop with calibrated test equipment. Without the proper test equipment, the altimeter must be removed and sent to an instrument shop for certification before the pitot-static system can be recertified.
Although 91.411 applies to airplanes that operate in IFR conditions, FAR 91.215 lists the airspace where an encoding altimeter is required. There's lots of it, and a properly operating and calibrated encoding transponder (which gets its altitude information from the static system) makes it a lot easier to go where you want to when you want to go. Although a few pilots might argue that there's no regulatory requirement for a biennial static system test, that argument ignores the reality of today's airspace. It's busy and getting busier.
Unfortunately, both Leoni and Mike Meyer of Instrument Tech ( www.instrumenttech.com) in Addison, Texas, said that improperly performed pitot-static system certifications are the biggest cause of pitot system instrument malfunctions. Putting positive pressure to the static side of the system or negative pressure to the pitot side exposes diaphragms to forces they were never designed for. Rapid changes in suction or pressure also take a toll. Although the diaphragms will perform for decades without problems, they're very sensitive to nonstandard pressure applications.
The pitot tube or mast should be given a good look during each preflight. Any misalignment or impact damage can cause inaccurate readings. Pitot tubes are inviting homes for flying bugs — put something over the hole when the airplane is on the ground. Some pilots buy a cover complete with a remove before flight flag while others install air-activated hinged covers. Some just cut a hole in a tennis ball and pop it over the mast.
Static ports and pitot tube holes should be covered during airplane washing. Any tape that doesn't leave a deposit can be used — one trick to help ensure that the tape is removed after the hose and brushes are put away is to put a foot-long flag of high-visibility surveyors tape on the covering tape. Surveyors tape is the neon orange or green plastic ribbon commonly seen at construction sites. It can be bought at a hardware store.
Don't hang your coat or anything else on the pitot mast or tube. And don't blow into the pitot tube or mast. If you suspect the plumbing is plugged, the plumbing must be disconnected from the instruments before back-blowing (blowing toward the tube or port) the system with compressed air.
The pitot-static system is pretty bulletproof, and the instruments are very reliable if handled correctly. If the face of your airspeed indicator is yellow and tired- looking, send it in for cleaning and recalibration — after all, it's probably been taking good care of you and your airplane for at least 20 years.
E-mail the author at [email protected].
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