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to “unstick” the needle. Analog instru- ments work a lot like bathroom scales in that changes in pressure mechani- cally 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 me- chanical 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 man- dated in January 2005 for U.S. air- space, there was a big push for own- ers 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 direc- tion 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.
A visual reminder is helpful in preventing out-of-configuration incidents and irregularities.
Modern air data computers have greatly improved accuracy and reliability in flight environment data.
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 con- solidated in the air data computers is that we see a high
level of reliability and ac- curacy. The downside is that when air data com- puters 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 espe- cially true here. There were two fatal 757 crash- es eight months apart in 1996 where a failure of the pitot/static sys- tem set into motion the events that caused both airplanes to crash.
Birgenair 301 sat outside in the Dominican Rep- ublic for 20 days. Two days before the accident flight,
March 2023 / TWIN & TURBINE • 15