It was a little after noon on a cloudy December day when a B36TC Bonanza with five aboard climbed out of Baker City, in northeastern Oregon, bound for Butte, Montana, 234 nm away. A few minutes after takeoff the pilot called Salt Lake Center to activate his IFR flight plan. He was cleared direct to the Donnelly VOR (DNJ), then via Victor 121 at 13,000 feet.
Fifteen minutes later, when the Bonanza was a few miles past Donnelly, the controller noticed that it was not on the airway. He provided the customary nudge: “N36ML, just confirm you’re level 13,000, established on the airway Victor 121.”
“Negative,” came the reply. “36ML’s picking up too much ice, ah, we’d like to divert to, ah, Salmon.”
“’K, everybody stand by. N36ML, confirm you’re level 13,000.”
The controller could presumably see from the Mode C readout that the airplane was below its assigned altitude.
“Negative, ah, 36ML’s, ah, would like to descend to 11,000.”
“N36ML, maintain 12,000 for terrain.”
The Salmon VOR was the next one along V121 after Donnelly; the airport, Lemhi County (SMN), lay a little to the east of it and about 88 nm, or 25 minutes, from the Bonanza’s present position. The pilot acknowledged the altitude assignment and the controller cleared him direct to the airport after crossing the VOR, repeating, as he did so, the 12,000-foot altitude assignment.
Three minutes passed. N36ML had now begun to drift north of the airway.
“N36ML, just confirm you’re still established on Victor 121.”
“Affirmative, ah, establishing Victor 121, 6ML.”
“N6ML, turn, ah, 15 degrees right to be established on the airway.”
“Roger, 6ML.”
“Low altitude alert, N36ML, check your altitude immediately. The minimum IFR in your altitude is 11,900; climb and maintain 12,000 immediately. Say altitude.”
“11,500, ah, climbing, 6ML.”
“N36ML, I cannot have you descend, sir; climb and maintain 12,000 immediately for terrain.”
“6ML’s having engine problems. … 6ML needs to go to 3U2 immediately.”
“N36ML, say altitude.”
“10,000.”
“N36ML, are you able to climb to 12,000?”
“Negative, 6ML.”
The airport whose code is 3U2 is Johnson Creek, a 3,400-foot turf strip in a narrow valley. Unattended and unmaintained in winter, flanked by ridges rising 4,000 feet above the runway, it was an unpromising haven for an iced-up airplane in IMC.
“N36ML, roger, McCall Airport is at your 6 o’clock, 24 miles; would you like to divert?”
“Affirmative, I’d like, ah, guidance in, 6ML.”
“N36ML, roger, suggest you fly heading 253 and just, ah, the minimum IFR altitude in your area is 11,900.”
“6ML just lost its engine, say again, ah, heading for 6ML.”
“N36ML, McCall Airport is at 6 o’clock, suggest heading 253 for McCall Airport. Additionally I have Johnson Creek Airport right below you, identifier is 3U2 and I have information for that airport if you’d like that information.”
That was all. The controller tried again and again to contact N36ML through relays, but nothing more was heard.
The severely fragmented wreckage, buried in snow, was found 40 days later by a determined private search party of the pilot’s friends and relatives. A handheld GPS preserved the record of the final flight. The pilot did not turn back toward McCall but tried for 3U2 instead. Based on the automated weather report for McCall, the cloud bases in the area may have been at around 7,500 feet msl. To glide down into the clear within sight of Johnson Creek without encountering any of the nearby 9,000-foot ridges would have required better luck than most people have, but the pilot came close. The Bonanza collided with a densely forested ridge at 7,600 feet, 1.6 nm east of the airstrip.
After recovering and examining the wreckage, the National Transportation Safety Board’s investigators concluded that the loss of power must have been due to induction icing, since nothing else was found that could have kept the engine from running. The B36TC has a spring-loaded alternate air door that opens automatically in the event of the filter freezing over and allows the engine to develop rated power up to 13,000 feet. If the alternate air door itself becomes jammed by ice, it can be forced open by pulling a T-handle under the panel. The alternate air door was found to move freely, but the actuating cable was broken close to where it attached to the door. Whether the break was due to the crash, the post-crash fire or something else is not clear from the accident report.
That there was no record of the pilot obtaining a weather briefing has become a regular refrain in NTSB reports, but it means little now that self-briefing is possible from so many sources. It is reasonable to suppose that a 1,000-hour pilot filing an IFR fight plan would get some kind of weather briefing. The question is, how thorough a briefing did he get and how carefully did he weigh it?
Multiple airmets warned of possible mountain obscuration, turbulence, moderate icing and low-level wind shear over a wide area. Cloud bases were expected to be around 8,000 feet, tops around 13,500 feet, and the freezing level 8,500 feet. The weather at the destination, Butte, was good. The wind at 13,000 feet was forecast to be 300 degrees at 53 knots, so the pilot could expect a quick trip and only brief exposure to icing.
From the evidence, it appears that shortly after entering the mountains the Bonanza picked up ice very rapidly — sufficient ice that in spite of being turbocharged it could not maintain altitude. If the accretion of ice then slowed or stopped, we can perhaps understand why the pilot chose to divert to Salmon, far ahead, rather than back to Baker City. If he had turned back to Baker City he would have had to fly through the area of heavy icing again while bucking a powerful headwind.
The combination of high, rugged terrain and a forecast probability of moderate icing raises questions about the decision to make the flight in the first place. Inevitably, the outcome influences our thinking. Other airplanes were aloft in the area that day, however, and there were a number of pilot reports of icing, but the other airplanes did not crash. On the other hand, when we weigh risk we are weighing possibilities, not certainties, and the possibilities were bad enough.
The B36TC was not equipped for flight in known icing. It was, however, turbocharged and theoretically capable of climbing to a level where icing would not occur. Most instrument-rated pilots have seen a little ice on their wings and windscreens and observed that their airplanes kept flying quite comfortably. The pilot may have reasoned that if he encountered icing he would be able to escape from it.
“Known icing” means observed icing; in other words, icing is not “known,” weather forecasts or pilot reports notwithstanding, until you see it on your airplane. There is no rule against flying into possible icing conditions. In a 2009 letter regarding the meaning of “known icing,” the FAA’s chief counsel noted that “area forecasts alone are generally too broad … to be particularly useful to a pilot.” On the other hand, the letter went on, pilots “should have a viable exit strategy and immediately implement the strategy” if they encounter icing. The lack of an exit strategy, or the failure to implement one the moment rapid ice accretion began, doomed this flight.
This article is based on the NTSB’s report of the accident and is intended to bring the issues raised to our readers’ attention. It is not intended to judge or to reach any definitive conclusions about the ability or capacity of any person, living or dead, or any aircraft or accessory.
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