The visual of large blue and green aluminum fragments floating in the ice-laden Potomac River as we descended on our approach into Washington National Airport is still vivid enough to remain locked in my memory. The evening prior, Air Florida Flight 90, a Boeing 737-200, had crashed into the 14th Street Bridge just after takeoff during a nasty snowstorm.
I remember the date because the ink hadn’t dried on my ATP certificate, my having passed the check ride only one day before. The dramatically documented accident occurred on January 13, 1982. Although many other significant factors contributed to the tragedy, the event became a watershed moment for the airline industry, bringing increased awareness to the dangers of airframe icing.
Most of us understand that icing deforms the surface of the wing, seriously degrading lift-producing capability. The degree to which the degradation occurs is dependent on many factors that include, but are not limited to, the type of precipitation, the intensity of the precipitation, the outside temperature and the shape of the airfoil.
Unfortunately, it wasn’t until just over 10 years after the Air Florida crash that deicing and anti-icing operations were profoundly improved. The improvement came as a result of the accident analysis of USAir Flight 405, a Fokker F28 that crashed shortly after takeoff from La Guardia’s Runway 13 during a snowstorm on March 22, 1992. What was discovered?
First, the National Transportation Safety Board determined that the Type I deicing fluid applied to the F28 on two separate occasions before taxi was not capable of continued icing protection for the conditions present beyond a few minutes. It was certainly compromised after the 35 minutes USAir Flight 405 spent on the ground. Type I fluid’s purpose is simply to rid the airplane of contaminants as a deicer, and has very limited ability as an anti-icer.
The solution was a new fluid called Type IV, designed to be applied after Type I fluid. The Type IV mixture is considered “anti-ice protection.” It has a much more robust ability to repel freezing precipitation (e.g., snow). The fluid is also designed to shear off the wing as the airplane accelerates during takeoff without creating an appreciable adverse effect on lift. The mixture is eventually compromised once it is saturated and can no longer absorb freezing precipitation. It has a time-limited useful life depending on the environmental conditions present. The useful life is called “holdover time.”
Holdover time is a subjective interpolation. Pilots are provided charts within their operation manuals as to a time period for when the breakdown of Type IV fluid occurs depending on the form of precipitation, precipitation intensity and outside temperature. If conditions change (i.e., precipitation intensity increases) the final authority for when holdover time is exceeded becomes the decision of the captain. On average, holdover times are in minutes, usually less than an hour.
Another improvement as a result of USAir Flight 405 was the establishment of icing-fluid stations strategically located near the departure end of the active runway rather than just at the gate area. This icing “car wash” is organized when weather conditions warrant it, the location reducing the holdover time required in freezing precipitation. The deicers themselves are FAA-certified and are authorized to evaluate the condition of the airplane. At most major U.S. domestic airports, each airline arranges for a specific ramp area, using company personnel or contract personnel to perform the deicing/anti-icing process.
Although the crew of USAir Flight 405 was cognizant of the potential threat that the snowstorm presented as they waited in line for takeoff, their evaluation of the airplane condition was not adequate. The crew didn’t have the appropriate procedural evaluation tools, nor did a lot of other airlines, for that matter. The captain did have the flight deiced with Type I fluid twice prior to taxi, but this was an easier situation, having the ability to view the condition of the airplane from outside at the jet bridge.
For all carriers, an operational change was required in the form of definitive procedures for contamination evaluation. I’m simplifying the procedure, but basically, if holdover time is not exceeded, then a cockpit check of items such as the condition of windshield wipers is considered. If the wipers have no evidence of frozen precipitation, then the takeoff can proceed. If holdover time is exceeded, then the airplane has to be deiced and anti-iced again. Or a certified deicer can make the evaluation. Or, in the absence of a certified deicer, a check is performed by a pilot from within the cabin because it is often difficult, if not impossible, to see the wings from the cockpit. Even with a cabin check, an evaluation at night can be problematic. (I speak from experience, having attempted to view accumulation on a B-757 wing during a nighttime snowstorm at JFK.)
Fast-forward to present day and the same basic procedures remain, albeit with a little tweaking. But now, the paper holdover charts are used as a backup instead. In their place is an iPad application that allows us to enter all the pertinent parameters to determine holdover time during any given freezing-precipitation event for each fleet type. We can even set a timer to alert us as to the end of the holdover period. The application eliminates much of the subjective interpolation of the paper charts, still leaving the final decision to the captain.
But alas, an even better mousetrap has been developed. Enter Vaisala, a corporation with headquarters in Finland and a main U.S. office in Boulder, Colorado. I spoke with Kevin Petty, the firm’s chief science officer. Kevin has an impressive resume that includes stints at the National Center for Atmospheric Research and the NTSB. One of the company’s many meteorological products is called CheckTime, a deicing application that has been in development for approximately five years.
Having agreements with various airport authorities throughout the world, Vaisala has been able to position sensors that measure a quantity called “liquid water equivalent” (LWE). What’s that? Imagine melting a snowball. The water remaining is LWE, a parameter that determines the effectiveness of any anti-ice fluid type for any given precipitation condition. The sensors can evaluate LWE every minute, continually updating holdover times even as precipitation intensity varies.
The CheckTime sensors send a message to the cockpit via ACARS or via internet connectivity through an iPad app. The message is a continual holdover-time update, an indication of when the anti-ice fluid will fail. The technology eliminates almost all of the guesswork, notwithstanding that it helps to eliminate canceled flights and delays due to additional anti-icing applications that would otherwise be unnecessary. The cost savings is in the thousands of dollars per airline per event.
Kevin indicated that Boston, Chicago and Denver all have the technology. Although he wouldn’t reveal other carriers, Emirates airline is one of the launch customers.
A phrase Kevin repeated during our conversation was “safety first through science.” I agree. It seems that science has created a better mousetrap that is better prepared to fight Mother Nature’s winter fury. Thank you, Vaisala.
