No Need For "What Ifs"

Air Safety Week, Jan 29, 2007

A Crash Investigator Gets A Break

When an NTSB crash investigator starts poking among the ruins of a burnt- out aircraft, he has a multiplicity of possibilities to eliminate. You'll always see early comment on the prop's or turbine's evident rotation at impact, the flight-control's continuity and cable routing, the position of a multitude of switches, the presence of fuel, the aircraft's configuration, and of course the post-mortem pronouncements on the pilot.

Nearly always these will relate a cumulative tale as he plods through his checklist. Much of that data will be negative or eliminatory, which is also useful. Two near-full fuel-tanks, a properly positioned fuel selector valve and evidence of engines turning at high RPM on impact is fairly conclusive, for instance.

However, some dead men tell no tales and it's almost impossible to say with certainty that there was carburetor icing, that there was water in the fuel, that there had been a pre-impact electrical fire and smoke, or that there had been carbon monoxide poisoning.

All the investigator can do is canvass the balance of probabilities, look at the developing scenario, take witness statements and weigh it all against the recorded R/T and the environmentals. Might the pilot have been disoriented or just fatigued beyond measure? It's a cumulative conundrum. But now and again an investigator gets a rare break.

On Feb. 16, 2005 a Cessna Citation crashed in freezing rain four miles from the Pueblo Memorial Airport Colorado, just after 9:00 a.m., killing all eight on board. The National Weather Service reported that there had been low cloud, fog and freezing drizzle. Those conditions are always a lead as to what may have happened, but investigators are crash detectives and trained not to be sucked in by the obvious.

N500AT was one of two company planes flying in consort from Richmond to Pueblo. An intermediate refueling stop was made at Columbia, Mo. The Citation 560 had received clearance to land runway 26R and continued its IMC approach at a rate of about 500ft/min towards a 900ft broken cloud-base (of 5 to 7 oktas), the overcast layer being at 1400 ft AGL.

Breaking cloud, it continued a rapid descent at almost 2800ft/min well short of the runway, impacting open ground left wing first and quickly burning out. Before the plane's sudden drop, "its descent profile looked normal," NTSB Investigator Frank Hildrup said. Its companion aircraft landed safely. Speculation centered on the Cessna's anti-icing system having been overwhelmed in the conditions. A voice recorder was removed from the other safely landed C560.

A March 25, 1998 AD (AD-98-04-38) had "revised the Airplane Flight Manual (AFM) to specify procedures that would prohibit flight in severe icing conditions (as determined by certain visual cues), limit or prohibit the use of various flight control devices while in severe icing conditions, and provide the flight crew with recognition cues for, and procedures for exiting from, severe icing conditions. ... prompted by results of a review of the requirements for certification of the airplane in icing conditions, new information on the icing environment, and icing data provided currently to the flight crews."

This AD would require flight crews to immediately request priority handling from ATC to exit severe icing conditions (as determined by certain visual cues). Additionally, it would:

* prohibit flight in severe icing conditions (as determined by certain visual cues);

* prohibit use of the autopilot (and limit use of flaps on approach) when ice formed aft of the protected surfaces of the wing, or when an unusual lateral trim condition existed; and

* require that all icing wing inspection lights be operative prior to flight into known or forecast icing conditions at night.

One commenter on the Notice of Proposed Rule-making accused the FAA of over-reacting to suspected and unsubstantiated "evidence" of an allegedly possible condition.

Nevertheless, the AD became a directive. The FAA had accidentally (you might say) "discovered" just how lethal SLD (supercooled large droplets) in freezing rain (aka, rain-ice) could be. It hits, sticks and destroys aerodynamics, all in very short order.

A similar crash of a Cessna 560 Citation V at Eagle River Airport, Wisconsin on Dec. 30, 1995 (N991PC) produced this probable cause:

"Failure of the pilot to maintain airspeed while executing the circling approach. Factors were the descent below minimum descent altitude, the fog, the low ceiling and the icing conditions."

While it now looks as if the N991PC cause was nailed, the role played by SLD icing was not. When an airplane stalls in severe icing conditions, it won't necessarily be the case that the pilot voluntarily descended (below min descent altitude) or "failed to maintain airspeed". With the right load of drag- producing ice, both propositions might just be aerodynamically impossible. Thus and therefore came the March 25, 1998 AD.