In the mid-18th century, French engineer Henri Pitot created a tube to measure the flow of water in the Seine River. German American inventor Paul Kollsman created the first accurate barometric altimeter in 1928. Pitot and Kollsman were separated by over 200 years, but the work of these two men has shaped the world of aviation.
Static pressure (Ps) is the pressure of the ambient air immediately surrounding an airplane and is measured by ports installed on both sides of the fuselage. Static ports connect outside air to the flight environment instruments and are placed in locations along the fuselage where the ambient air is unaffected by the speed, attitude or angle of attack of the airplane.
Pitot pressure, also called total pressure (Pt), is simply ram air forced into the pitot tube when an airplane moves through the air. The tube that Henri Pitot created in the 18th century closely resembles what we use in airplanes today to measure airflow. But what that tube connects to has evolved and been refined to produce an extremely accurate measurement of how fast we’re moving through the air.
In older pitot/static systems, there were tubes and hoses that provided static and pitot pressure directly to analog flight environment instruments. The only electrical connections were for backlighting and an internal vibrator for the altimeter so it wouldn’t have to constantly be tapped to “unstick” the needle. Analog instruments work a lot like bathroom scales in that changes in pressure mechanically move the needles, and dozens of feet of hoses, tubing and fittings were prone to chafing or cracking. Once a leak or an erroneous instrument got bad enough, it would usually manifest itself as an altimeter split between the pilot and copilot side with no way to know which side was the “bad” side.
Even with all the leaks and mechanical components of older air data systems, airplanes in the flight levels were vertically separated by 2,000 feet. So, even with the loose tolerances, the analog systems were enough to keep airplanes safely apart. Once the FAA declared that RVSM (Reduced Vertical Separation Minimums) would be mandated in January 2005 for U.S. airspace, there was a big push for owners and operators to upgrade to more accurate and reliable systems.
RVSM changes aircraft vertical separation requirements between FL 290 and FL 410 from 2,000 feet to 1,000 feet. Anyone who has ever watched an airliner pass a thousand feet above their cockpit windows in the opposite direction surely has a great appreciation for just how little of a margin a thousand feet is when the closing rate is over a thousand knots.
RVSM-capable flight environment systems use pitot and static pressures plumbed directly to air data computers (ADCs), which eliminates the vast majority of tubes and hoses that used to snake behind the instrument panel. Outside air temperature, barometric correction, and angle of attack data are also sent to the ADCs, where they’re analyzed, and a digital signal is then sent to the display via a digital data bus. A twisted, shielded pair of wires have replaced tens of feet of hoses and tubes, as well as dozens of leak-prone fittings.
In order to fly IFR in the U.S., an aircraft’s transponder and air data system have to be tested per FARs 91.411 and 91.413 every 24 months by a certified repair station. The upside of all the flight environment data being consolidated in the air data computers is that we see a high level of reliability and accuracy. The downside is that when air data computers break, it’s usually an expensive fix.
To keep your system running smoothly, a good preflight goes a long way. I know I say that about everything, but it’s especially true here. There were two fatal 757 crashes eight months apart in 1996 where a failure of the pitot/static system set into motion the events that caused both airplanes to crash.
Birgenair 301 sat outside in the Dominican Republic for 20 days. Two days before the accident flight,a maintenance crew removed all the protective covers to perform engine runs and neglected to reinstall the covers. On the takeoff roll, the captain’s airspeed indicator showed zero knots and only started “working” when they were airborne and climbing. What the captain thought was airspeed was actually the sea-level air trapped in the pitot line expanding against the now-decreasing static air and giving the false indication of positive pressure coming into the pitot tube. Several wrong assumptions later, the airplane crashed into the Atlantic. The pitot tubes were never recovered, but the most likely cause is that a mud dauber wasp built a nest inside the perfectly cylindrical tube.
In the case of Aeroperú 603, the crew neglected to find tape that covered some or all of the airplane’s static ports. The aircraft was washed before the flight and the ground crew didn’t remove the tape. The aircraft took off after midnight from Lima, Peru. The combination of erroneous airspeed and altitude indications and a lack of ground references on a dark night over water caused the aircraft to crash into the ocean.
When performing your preflight inspection, look inside the pitot tube for obstructions, preferably with a flashlight, and also look at the vent hole(s) on the outside of the tube to make sure there aren’t any clogs. If you see something inside the tube, unless it’s right on the edge of the opening and you’re sure it’ll come right out, call a maintenance tech. People have tried to dig things out of pitot tubes only to end up packing them like a Civil War musket. If you accidentally shove something into the pitot tube, tools, testing and invoices will likely be involved before you can go flying. It may look like a simple thing but remember that anytime even one fitting is loosened on the pitot/static system, leak checks and functional tests must be accomplished.
I know I don’t need to say this but humor me. Pitot tubes are not grab handles to use to get a good look inside the nose gear well. My knees hurt too, but please find a different way to brace yourself. Another pro tip from the “been there, got the scars” file: It only takes once to grab a hot pitot tube before you’ll forever lightly hit the tube with the back of your fingers before touching another one. I’ve done the “that might be hot” smack to a pitot tube that was lying on my toolbox before picking it up.
All the same advice applies to static ports. You typically can’t see very much in there, but it’s good practice to take a look anyway. On RVSM-certified aircraft, it’s important to keep the “RVSM critical” area around the static ports and pitot tubes clean because contamination or deformation in these areas can cause erroneous indications. Your aircraft’s AFM or POH should have a diagram of where the RVSM critical area is. Otherwise, contact the manufacturer or your favorite maintenance technician and they should be able to help.
Covers for your pitot tubes and static ports are as important as performing a good pre-flight inspection. Covers are an inexpensive way to add a lot of protection from bugs nesting in those perfectly round, perfectly bug-sized openings. You may feel the urge to roll those long red streamers up to make them look a little neater and to keep them from scuffing up the paint. Don’t do that. The sole purpose of those streamers is to be as obtrusive as possible to minimize the risk that they’ll accidentally be left on the airplane. There’s also very little chance that they’ll scuff the paint.
If you wet wash your airplane, take proper precautions when covering pitot/static probes and sensors. There is typically a procedure in Chapter 20 of your aircraft’s maintenance manual that will tell you how to properly wash your aircraft. If you don’t have access to your maintenance manuals, call your favorite shop or the manufacturer and ask if they’d provide you with the procedure. These precautions are important not only to protect the pitot/static system but to protect the airplane from ingesting water into places that can’t shed moisture, creating a higher-than-normal chance for corrosion.
If you use tape to cover your static ports for any reason, my recommendation is to include a couple of feet of fluorescent orange or pink surveyor flagging tape so it’s flapping in the wind and extremely hard to miss on even the most blurry-eyed pre-flight inspection. I use flagging tape any time I change the configuration of the airplane – pulling an inspection panel, disengaging a circuit breaker, or anything that needs to be reset before a flight. Because I work alone most of the time, it’s really easy to get sidetracked and forget about that engine cowling that is only secured with one fastener. Flagging tape is available at any hardware store and is an inexpensive visual reminder that something is outside its normal configuration.
A properly maintained pitot/static system is mind-bogglingly accurate. It will keep you safely away from all that other aluminum zooming around in the flight levels, but these systems are as delicate as they are robust. Something as benign as a piece of tape or an unfortunately placed wasp can cause chaos, but with proper maintenance and thorough pre-flight inspections, we can eliminate most of the “gotchas” that can foul up our most critical instruments.
