Water in the Static Air System
To my surprise, I learned the hard way this week that the static
air system in the Cirrus can apparently take on water in very windy IMC
situations. This presents a difficult set of choices, especially if and when
those big red X’s appear on the PFD. Switching to alternate (cabin) air will
likely result in an artificially high altimeter reading, which can be equally
Earlier this week I was flying in the evening from Highgate, VT (FSO) to White Plains, NY (HPN) after an Angel Flight mission. All but the first 20 minutes of flight was in IMC, and for the last 20 minutes the conditions were heavy rainfall, a 60 knot headwind and periodic windshear during the descent from the final approach fix. The weather at HPN was 1.5 miles visibility, 300 foot ceiling, wind 110 degrees at 15 knots gusting to 26, with a low level windshear warning. The ILS 16 approach was in use. OAT was not far above freezing, so as a precaution I turned on the TKS system while still about 20 minutes flying time from HPN.
Given the strong winds, the long runway at HPN and the jet traffic both ahead and behind me, my strategy was to maintain high speed on the approach, come over the numbers at 125 knots and land without flaps. I’ve done this many times at various big city airports, so I am comfortable with the power settings and landing distance needed to accomplish this.
I was at the assigned altitude of 4,000 feet when ATC cleared me for the ILS 16 approach and instructed me to intercept the localizer at FARAN intersection (15 miles from HPN). I agreed to maintain 160 KIAS for the 8 mile distance from FARAN to HESTR (the outer marker). Once established on the localizer, I descended to 2,000 feet so that I would be able to intercept the glideslope just past HESTR. After leveling off there was strong rainfall and occasional turbulence: it was then that the airspeed went into the yellow zone, in the 175-180 KIAS range. I did not think that the plane was capable of flying that fast with just 24 inches MP, and did not realize at the time that the airspeed was unreliable because water had apparently entered the static system. The PFD did not show any red X’s over those instruments, because there wasn’t enough water (or ice) to totally block the system. Both the PFD and analog instruments gave the same readings.
Rather than fly at a supposedly excessive speed, I throttled back to 20 inches MP and advised ATC that I was unable to maintain 160 knots. The airspeed now read 135 knots, but it occasionally varied between 110-140 knots, which I ascribed at the time to wind shear. ATC advised the jet behind me that they may have to slow down if the spacing between us decreased, but that did not prove to be necessary. I crossed the outer marker and was cleared as number three to land, with two jets ahead of me.
I intercepted the glideslope just past HESTR and flew a coupled approach with power at 13.5 inches MP even though airspeed was swinging all over the place, because I was comfortable with that setting. I broke out of the clouds at 1.5 miles and 400 feet above ground level, fought with some more low level wind shear and landed uneventfully. Again, the airspeed indicator was all over the place so I just kept the nose down and applied whatever power was needed to reach the runway.
The plane was hangered and plugged in for the night. The next morning was clear and cold; I started up the engine and the PFD showed three big X’s where the airspeed, altimeter and VSI readouts should be. I pulled the circuit breakers several times and restarted the plane, but each time got the same result. I spoke to tech support at Cirrus, and they felt that the fault was probably blockage in the static system rather than a defective PFD. Sure enough, as soon as I switched to alternate (cabin) air the PFD data came alive. I did a high speed taxi on the runway to confirm that both the PFD and analog instruments were functioning using alternate air, and then flew in VMC to Lincoln Park for maintenance of this problem.
At Lincoln Park the mechanic opened the access panel to the static system drain, but there was no visible water accumulation: apparently the water was somewhere else in the tubing. He blew out the system, and a considerable amount of water vapor could be seen coming out of both external static ports. He then did a leak check and certified the plane as airworthy. I then took off (on an IFR flight plan) for State College, PA (UNV).
About 20 minutes into the flight, however, I realized that something was amiss, even though both the PFD and analog instruments showed the same data. By this time I was up at 6,000 feet in VMC and the OAT was –4 degrees centigrade. What tipped me off was once again the airspeed: it was in the yellow zone, at normal power in level flight. I powered back and the nose tipped down, but the altimeter and VSI were frozen at 6,000 feet and zero, respectively. Now I knew that the strange airspeed indications the previous evening and that day were due to a serious static system problem, so I declared an emergency and advised ATC that I would be returning to Lincoln Park. After reversing course I flipped the alternate air switch, and the altimeter showed that I had descended almost 1,000 feet.
This morning the static system was cleaned out again and the plane returned to service. Apparently there were additional drops of moisture in the bends of the tubing near the tail which had not flushed out yesterday. I took a test flight this afternoon and everything worked fine. I did note that the altitude shown on both the PFD and analog instrument using alternate (cabin) air was 100-120 feet higher than normal, even with the vents open.
In retrospect, it is apparent that the static system picked up water during the heavy rainstorm on the approach to HPN, but not enough to make a failure evident (e.g., by the PFD showing three red X’s). I did not begin to appreciate how insidious this condition could be until yesterday afternoon’s flight, when I got the same weird airspeed readings under relatively calm visual conditions. In the thick of a difficult IFR approach, I had ascribed the telltale airspeed problems to varying wind conditions, but chose to do a coupled approach with known power settings to make the descent.
I now realize that if I had gotten the red X’s on the PFD during the approach and shut down that system and flown a GPS approach (as taught by UND) I would probably have been doing so with an inaccurate analog altimeter (since it is fed by the same static system as the PFD). Since I was flying into a towered airport with lots of other traffic, I had some comfort that I would break out above minimums and could rely on the glideslope for altitude guidance.
Equally significant is the issue of potentially misreading the PFD error message. When those red X’s appear in flight, is it a defective PFD or a blocked static system? And if the system is blocked, then one faces a difficult choice in turning off the PFD and thereby losing its HSI and ILS capabilities, because now you risk having no reliable altimeter information.
Alternatively, if one correctly identifies this problem as blockage in the static air system and switches to alternate (cabin) air, one needs to be very cognizant of the fact that the altimeter reading is probably higher than actual altitude. This can be quite significant if one is navigating low to the ground or attempting an approach at minimums with no advance knowledge from other pilots as to what the actual conditions might be.
One final point: I don’t know how the design of the static air system in the Cirrus compares to other aircraft, nor do I know what the FAA rules say about the testing and certification of these systems. But it seems to be a troubling situation if water can enter the system in a tough IMC situation, and one is left with a series of difficult choices at precisely a most inopportune time. Also troubling is the fact that there is no clear indication that the static system is becoming blocked (unlike a vacuum pump airplane, where there is a gauge to show declining performance).