The Boeing 727-200, operating as Northwest Airlines Flight 6231, departed John F. Kennedy International Airport in New York, N.Y., at about 1926 Eastern time on December 1, 1974. A ferry flight with only crew aboard, the 727’s destination was Buffalo, N.Y., a great-circle distance of 261 nm. After takeoff, the aircraft climbed to 13,500 feet msl and leveled off for about 50 seconds, accelerating from 264 knots to 304 knots. It then began to climb, at 2500 fpm, still at about 305 knots. As the jet climbed out of 16,000 feet, airspeed began increasing, followed by rate of climb. According to the NTSB, at about this time, the First Officer said, “Do you realize we’re going 340 knots and I’m climbing 5000 feet a minute?”
The airplane’s climb rate soon exceeded 6500 fpm and an overspeed warning horn sounded at 23,000 feet, with airspeed at 405 knots. Shortly afterward, the stall warning stick shaker was recorded intermittently and, five seconds later, vertical acceleration reduced to 0.88 G. The airplane briefly leveled at 24,800 feet and an airspeed of 420 knots. About 13 seconds later, the airplane began descending at 15,000 fpm. The jet’s magnetic heading changed from 290 degrees to 080 degrees within 10 seconds. The 727 had stalled and was spiraling down. It descended more than 23,000 feet in 83 seconds, striking the ground about 3.2 nm west of Thiells, N.Y. Parts of the horizontal tail were located up to 4200 feet from the main wreckage. There were no survivors.
What can make a lightly loaded 727 stall and spiral down to the ground without its crew understanding what was going on? Iced-over pitot tubes.
Planning For Pitot-Static Problems
Now, while you’re on the ground, is a good time to prepare and plan for a pitot-static system failure. Consider the following :
Locate and identify the airplane’s alternate static source valve (it’s likely a valve; it could be a switch). The airplane’s documentation (POH/AFM) should include a discussion of how to operate the valve/switch and what happens when you do (e.g., indicated airspeed changes, etc.) Next time you’re out on a decent VFR day, activate the alternate static system and note what changes.
Pitot Heat Switch and Breaker
Same thing with the pitot heat: Locate and identify the system’s switch, any associated indicator lights and the relevant circuit breaker. Although the POH/AFM may include a system discussion, there’s little to learn there and likely no changes to your operating procedures when it’s activated. You can check it in flight in most airplane by turning it on while watching the ammeter, which immediately should indicate a higher overall amperage. Minimize ground use, including during preflight—it can get hot.
Pitch + Power = Performance
What if you lose your airspeed indicator? If you’ve done a little bit of planning and maybe some test-flying, you’ll know what power settings and pitch attitudes to use in getting back on the ground. For example, 20 inches and 2400 rpm might get you 130 KIAS in straight and level flight. Seventeen inches and three or four degrees nose-down with the landing gear extended might ballpark a three-degree glidepath. Going around at full power means pitching to five degrees nose up for, say, 100 KIAS and a 750 fpm climb. And nailing these numbers and more is a fun way to spend some practice time.
The NTSB determined the accident’s probable cause was “the loss of control of the aircraft because the flightcrew failed to recognize and correct the aircraft’s high-angle-of-attack, low-speed stall and its descending spiral. The stall was precipated (sic) by the flightcrew’s improper reaction to erroneous airspeed and Mach indications which had resulted from a blockage of the pitot heads by atmospheric icing. Contrary to standard operational procedures, the flightcrew had not activated the pitot head heaters.”
As accident-related lessons go, this one is fairly simple: Turn on pitot heat and other anti-icing system designed to ensure continued operation of flight instruments and other critical systems when flying near, at or below freezing temperatures. What apparently stymied this crew and prevented them from understanding what was going on was the lightly loaded airplane. It’s likely an empty airplane was something they rarely experienced, and they were quite pleased with the performance boost. They didn’t stop to question whether what they were seeing on their instruments was real.
When the 727’s pitot tubes iced over, the drain holes also became plugged—the airplane’s static sources remained unblocked—and the airspeed indicator (ASI) effectively became an altimeter. Instead of displaying airspeed, it sensed changing pressure. As the jet climbed, ambient pressure decreased, allowing the diaphragm within the ASI to expand and display increasing airspeed. If the crew had leveled off and noted the ASI, they would have seen it remain steady. If they initiated a descent, the ASI would have shown decreasing airspeed.
To fully understand the instrument indications caused by an iced-up pitot tube, we have to understand how an ASI works, which is by sensing the difference between ambient pressure obtained at the static port and the dynamic pressure created when fast-moving air enters the pitot tube. That difference is what’s registered on the ASI. But a blocked pitot tube is only one pitot-static system failure mode we might encounter. And the others also can be caused by ice on the airframe.
Blocked Static Port
Now, let’s reverse the scenario the 727’s crew encountered by blocking the static port(s) but leaving the pitot tube clear and operating. What will happen, and how will you compensate?
First of all, the ASI will continue to function but will be inaccurate. The ASI is receiving dynamic-pressure air from the pitot tube, but the yardstick against which the instrument measures that pressure is trapped and cannot change. As long as the altitude at which the static port became blocked is maintained, the ASI will indicate as if there is no blockage. But if you change altitude—presuming a constant airspeed—the ASI will indicate an airspeed dependent on the relative pressure difference it senses. Because the static system pressure is constant and based on the altitude at which it became blocked, the ASI will indicate a higher airspeed at a lower altitude and a lower one if you climb.
Put another way, at a lower altitude, denser air is entering the pitot tube. Because the ASI is sensing greater dynamic pressure than at the higher altitude where the static system became blocked, greater indicated airspeed on the ASI is the result. Of course, the converse is true: In the same blocked-static scenario and the same airspeed, the relatively reduced dynamic pressure entering the pitot tube will be compared to the static system’s constant pressure and tell you you’re flying slower than before you initiated the climb.
Meanwhile, the ASI isn’t the only instrument depending on the static system for information. The altimeter and verticial speed indicator VSI also will be affected if the static system becomes blocked. Since neither the VSI nor the altimeter depend on any source for their operation other than the static system, what will happen? With a blocked static system, the altimter will be “stuck” displaying the altitude at which the blockage occurred. The VSI will indicate zero rate of climb throughout.
Just because you have an alternate static system and activate it, it doesn’t mean there is no operational impact. Your airplane’s AFM/POH will have the details, but it’s likely you’ll still be looking at instrument errors, although they’ll be much more benign. The errors are introduced into the system because the pressure inside the cabin is slightly lower than ambient. That’s due to the venturi effect of pushing the cockpit through the air at whatever knots you can muster.
A slightly lower-than-ambient static pressure affects your instruments by fooling them into thinking the static system’s air pressure is less than it is. Since the altimeter converts that ambient pressure into height above sea level, the altimeter will indicate a slightly higher altitude than normal. Similarly, the VSI will indicate a climb, but only momentarily. Then it will settle down and should indicate normal rates of climb and descent.
Finally, your ASI on alternate static will indicate a slightly higher airspeed than would be the case if operating on the normal static system. The reason the ASI reacts this way is the same as before, when we had a blocked pitot tube or a blocked static port: The ASI senses the difference between ambient and dynamic pressure. Since the ambient statuc pressure inside the cockpit is slightly lower than actual, the difference measured by the ASI will be greater and it will “reward” you by displaying a faster airspeed.
A final word on alternate static systems: Your airplane may not have one. It used to be alternate static systems weren’t installed on basic airplanes, presumably because their manufacturers didn’t think they’d ever be routinely used for instrument flight. So why bother? The good news is that if your airplane doesn’t have one, an alternate static system source likely can be installed. All it should take would be an appropriate valve placed in the static line between the external port and the first instrument in the circuit. Appropriate placards, certification basis and paperwork still apply, of course.
In lieu of installing an alternate static source, old-timers were taught to break the glass on the VSI. This opens up that instrument and all to which it’s connected to the static air inside the cockpit. As we’ve discussed, the venturi effect means this air will be at a slightly lower pressure than ambient.
Now that you know how the pitot-static instruments will react to the two types of blockages they can suffer, how will you react? More important, how will you recognize, say, a pitot blockage for what it is? Will you do better than the crew of Northwest Flight 6231? You will if you have a solid understanding of the performance you can expect from the airplane you fly. The basic idea is covered more fully in the sidebar on page 21, but it all comes down to setting power and pitching the airplane for the desired performance.
Recognizing a pitot-system blockage can be difficult. But if the ASI is displaying speeds higher than should be possible given the airplane’s pitch and power configuration, that should be your first clue. Increasing airspeed while climbing and decreasing airspeed while descending just ain’t right, and you should act accordingly. Ensure pitot heat is on and the circuit’s breaker is closed. Cycle it if you’re not sure it’s working, and look for a changed indication on the ammeter. If it can’t be cleared, alternate static won’t help, and you may have to land without an accurate ASI. Now’s not a good time to be planning to land on a short field; go someplace where an extra few knots won’t hurt.
Jeb Burnside is this magazine’s Editor-In-Chief. He’s a 3200-hour airline transport pilot and aircraft owner.