Like the plumbing in aging homes, aircraft pitot static systems are among the most neglected systems in the aircraft. And neglect often means expensive repairs and even safety-of-flight issues. It doesn’t have to be that way because there are regulations in place to keep tabs on the system. Trouble is, too many aircraft owners put off the two-year trip to the avionics shop for a checkup and it’s a setup for hefty repair bills. Know the regs and understand the pitfalls of falling behind the upkeep.
Know the Regs
FAR Part 91.411 and 91.413 govern that for flights in controlled airspace under IFR, all aircraft (including experimentals) must have their altimeter system and transponder equipment inspected and tested in accordance with the instructions contained in FAR Part 43, Appendix E (altimeter) and F (transponder) within the last 24 calendar months. That’s hardly new. Avionics shops all over the country have been performing these necessary inspections for years at rates that have been steadily increasing. Plan on at least $350 to $500 for the average single or light twin-engine airplane and a lot more for pressurized aircraft. You’ll also pay more for multiple altimeters. For most shops, this is filler work, and most see the service as a required burden that makes extra money on top of installs. But the real bread and butter is when the shop finds a problem, and that’s what big teardown and big invoices are made of.
Compliance with Part 91.411 is broken out into two separate functions. The regulation states that the altimeter as a unit and any altitude reporting equipment shall be inspected every two years and must pass a series of tests outlined in Part 43, Appendix E. These inspections are to be carried out by appropriately rated individuals or instrument shops/Repair Stations approved for such work. There’s also required calibrated test equipment that needs to be maintained. The second required inspection is a test and inspection of the static system, which includes, among other things, a system integrity test, more commonly known as a leak check. It is this system, not the altimeter (especially in a day of electronic instruments), that generally fails the altimeter/ transponder check, and repairs can be time-consuming and costly. Fortunately, the static leak check can be performed by your local A&P mechanic or, better yet, by you, under the supervision of your local A&P mechanic. Either way will save some money when the tech comes to hook up his test equipment.
What Goes Wrong?
The average single-engine airplane has around 30 feet of plastic or metal static system tubing, broken by endless fittings and adapters all designed to create an airtight loop, which ties the appropriate equipment to a single ambient static source (or dual port with IFR aircraft). And just because the aircraft has been upgraded to electronic flight displays doesn’t mean the static system is less important. In fact, it’s more important than ever because these digital systems are plumbed into the static system and are less tolerant of leakage (and blockage) than mechanical flight instruments.
The purpose of the static system, of course, is to provide instrument sensing of a change to outside ambient pressures, which translate, ultimately, into some altitude diversion. Except for an alternate static source, the system has no moving parts. It is simply a loop made from plastic tubes and equally plastic flareless fittings, which are supposed to allow flexibility around the back of instrument panels and through fuselage formers and bulkheads. The design of the system requires a water separator to be installed at some low point in the line and the hardware must be of sufficient quality to resist kinking at bends and remain free from corrosion. It’s simple, and yet, every other year or so, a new leak is found or some fitting has worked loose due to vibration or who-knows-what.
Maybe a carpet screw was too long or driven into a side panel slightly off the mark, drilling the static line tucked behind the forward fresh air vent, or maybe the VSI line worked loose when that airport delinquent who helped you bleed your brakes managed to squirt 5606 all over the back of the panel. It is for this reason that checking the static system yourself, prior to getting the altimeter and transponder checked, will save you money, time and aggravation.
Pay attention to signs of trouble. If you see condensation on the bezel glass of round-dial pitot static instruments, there’s a pretty sure chance the static system has water intrusion and may be wide open. Protect the system by using pitot covers and avoid spraying water into the static ports on the fuselage when washing the aircraft.
But I Don’t Fly IFR
This really isn’t a good excuse for not accomplishing 91.411 testing and certification. Remember, the regs say that for VFR ops you only need to comply with 91.413, which checks the transponder. And these days, the tech should test the ADS-B Out function in mandate-compliant aircraft. Think in terms of preventive maintenance because even if you do only fly VFR, do you want to operate with a substandard pitot static system? Of course not. Plus, if you eventually decide to start flying the aircraft IFR you might be faced with substantial repairs because the system hasn’t been maintained.
Incrementally, dealing with minor repairs every couple of years (replacing one or two cracked fittings, condensation sumps and even small runs of static line) will be cheaper than having to open up the entire airframe to figure out where the system is leaking when the obvious areas don’t turn up any clues. If you park the aircraft outside in harsh climates, the system (especially ones made of plastic lines and fittings) may be more susceptible to problems as the lines become brittle and crack from major fluctuations in temperature.
Last, if you’re buying an aircraft that is being represented as IFR capable, look carefully at the logbooks to make sure FAR 91.411 and 91.413 have been complied with and are current. While you’re at it, look for signs that the system has been maintained and that these checks have been accomplished on a regular basis. Voids in certification should be red flags that you could be in for big repair bills once the airplane is yours.
This article hits hard right now. The Black Hawk collision at DCA shows exactly why keeping your pitot-static system in shape matters. That helicopter’s altimeter was reading nearly 100 feet high, and a few others in the same unit had similar issues. That kind of error comes from leaks, moisture, or fittings that loosen over time, stuff that 91.411 and 91.413 checks are supposed to catch.
To make it worse, the Black Hawk had its ADS-B shut off, so ATC and other aircraft couldn’t see where it was or how high. And even if ADS-B had been on, it allows for up to 125 feet of error in altitude squawk. So in this case, it probably wouldn’t have helped much anyway.
And that’s assuming the rest of the system was working right. But it wasn’t. The helicopter was flying its assigned route,Helicopter Route 4, now decommissioned, which had a 200-foot upper limit, running perpendicular and under traffic landing on Runway 33. The vertical gap between them? Just 75 feet. This article nails why those inspections matter. It’s not about paperwork.