When Flying in Ice

When it comes to flying ice-capable equipment, the bottom line is: If youre gonna do it, do it right


On Valentines Day 2000, Indy racer Tony Bettenhausen went down in a Baron 58 that was approved for flight in known icing conditions. FAA records show that in mid-afternoon the day before his fatal accident, he called Nashville Automated Flight Service Station to file an IFR flight plan and get a standard weather briefing.

He was told that along his route of flight there was a convective Sigmet reporting a line of thunderstorms 30 miles wide, with tops to 30,000 feet. In addition, the forecast called for moderate rime and mixed icing from the freezing level to 24,000 feet, with moderate turbulence below 15,000 feet. He updated the weather later, then wisely decided to cancel the flight.

The next day at 08:00 hours, he filed an IFR flight plan from Bristol, Tenn., to Indianapolis. The briefer told him to expect occasional moderate rime or mixed icing below 10,000 feet over Indiana and western Kentucky. Weather was IFR along his entire route of flight. The briefer stated youre going to get some ice for sure.

The Pireps he was given were innocuous. A Boeing 727 reported moderate rime ice from the surface to 3,500 feet; an MD 80 reported that during climb-out they encountered moderate icing from 4,000 feet to 6,000 feet. Another MD 80 reported light chop and light rime icing at 10,000 feet.

The briefer added, Theres a lot of ice up there. The pilot responded, Weve got boots and well be all right. It will improve in the next couple hours as well. The briefer agreed, stating, It looks to me like that stuff should blow out of there. Thus the pilots decision to depart was perfectly logical.

The pilot and three passengers were airborne at 10:35 a.m. For some reason instead of climbing to on top conditions at 12,000 feet, he was cruising the Baron 58 in the clouds at 10,000 feet.

At 11:40 he advised ATC that he was picking up ice at 10,000 feet and requested clearance to 12,000 feet. Within 25 seconds he was cleared to the requested altitude. About 55 seconds later the pilot stated that icing was pretty steady and pretty moderate at 10,000 feet. He added that the windshield and wings were covered with ice. Then he reported being on top at 10,500 feet. This was consistent with the Pireps he received from his FSS weather briefing.

This low time pilot – 511 hours total time with 58 hours of instrument time and 12.8 in the Baron 58 – attempted to zoom up to his newly assigned altitude. His speed as measured from the radar track dropped rapidly from 145 knots.

The airplanes POH has a minimum recommended airspeed of 130 knots for flight in icing conditions, but by the time the airplane reached 11,200 feet, his groundspeed had decayed to 85 knots. While a power-on stall speed is not provided, the idle power stall speed (at 5400 pounds gross weight) is 82 knots. Wing ice, of course, would increase the stall speed.

From 11,200 feet, ATC radar showed the airplane began descending, with the last plot at 3,800 feet. The aircraft crashed less than 800 feet horizontally from where it began its descent. While the accident report was notable for not stating the obvious, the damage description is indicative of a flat spin to the left. The left wing and cabin were consumed by fire.

Pilot Technique
Consider the pilots preparation. The accident investigation found that his multi-engine flight instructor cautioned the pilot about the weather and volunteered to come to Bristol for the flight home. The pilot declined and said he would wait for the weather to improve.

On the day of departure, his decision to go was completely rational, based on his instrument rating, recent training and experience, combined with the aircraft equipment. However based on the Pireps of icing and cloud tops at 11,700 feet, his selection of 10,000 for a cruise altitude is questionable. (A Hawker reported tops at 9,200 feet, but the Baron 58 was flying physically in IMC.)

Despite being icing certified, the Baron 58 POH states These aircraft are not approved for extended flight in moderate icing conditions or flights in any severe icing conditions. Thus his choice of a cruising altitude in clouds with reported ice is questionable.

As for the windshield being covered in ice he should have had the electrothermal anti-ice system ON during climb and cruise. It is approved for continuous operation and he knew to expect ice.

The investigators did not speculate as to whether he actually cycled the de-ice boots. However, by the date of this crash Raytheon had corrected the traditional recommendation to allow ice to build up before booting it off. This was meant to avoid ice-bridging, but tests show bridging is not a factor with modern boots. The pilots loss of control at 85 knots implies that wing leading edge ice contamination was probably relatively minor.

The Mental Angle
Psychological problems may have come into play at this point. When climbing through IMC with the clouds getting lighter, and with the sun (or moon) beginning to show through, it is natural to anticipate breaking out on top of the overcast.

New and inexperienced instrument pilots often unwittingly pull back on the controls to expedite the event. (Note that this also occurs when breaking out during an instrument approach, and the pilot abandons the approach procedure and attempts to duck-under and find the runway visually.)

In this case the pilot was likely apprehensive about the ice buildup. This no doubt intensified his desire to zoom-up out of the problem. Yet the Barons POH states very clearly that the minimum airspeed during icing conditions should be 130 knots indicated. In addition, it is never proper when cruise climbing to zoom up in such a fashion that the airplane ends up close to stall speed.

The reason for the minimum flight speed is to keep the ice from forming on the boot-protected leading edges of the wing and tail. The Baron POH states, The pilot must be aware … that if he allows airspeed to deteriorate below this minimum speed, angle of attack increases to the point where ice may build up on the underside of the wing aft of the area protected by boots. … Under some conditions it may even build up aft of the boots despite maintenance of the prescribed minimum speed.

The Baron pilot reported his windshield and wings covered in ice. This implies freezing drizzle or so-called mixed conditions, which involves a moisture droplet size with liquid water content much greater than the 40 to 50 microns used for airplane icing certification. In other words, that means severe ice.

The Baron POH stipulates that continued flight in these [mixed] conditions is extremely hazardous, regardless of the performance of the de-ice/anti ice system.

This apparently motivated the pilots request for an altitude change. Then with everything going his way he used an inappropriate zoom-climb that led to the airplane stalling and spinning to the ground.

The Legal Jungle
While pilots often gripe about the FARs, they do have value in providing certain guidance in decision making. Commercial pilots flying under FAR 135 (air taxi) or FAR 121 (scheduled airlines) not only have the regulations but also operations specifications, which provide detailed guidance for decision making. Most GA pilots dont realize that for light airplanes – 12,500 pounds or less – there are no FAR limitations for flight into icing conditions. The only reference source is the Limitations section of the POH.

For example the Baron 58 supplement states Beech Kit 58-5012 properly equips the airplane for flight in icing conditions. Cessnas 414A POH allows …IFR operations and flight into icing conditions as defined by the FAA. Alas the FAA fails to define in-flight ice. For example mixed conditions involves moisture droplets much larger than required for certification. Freezing drizzle or rain always involves moisture droplets much larger than system certification requirements. Thus these conditions constitute severe icing.

The Aeronautical Information Manual gives the pilot guidelines for reporting light, moderate and severe ice. Moderate icing may create a problem if flight is prolonged. Occasional use of de-icing/anti-icing equipment removes or prevents accumulation. Severe icing, it tells you, involves in-flight icing greater than the anti-ice/de-ice system can handle.

Now comes the confusion. You distinctly remember reading about icing limitations in FAR 91. And its true. Except that FAR 91.527, Operating In Icing Conditions, applies only to Large and Turbine Powered Multiengine Airplanes. Worse yet, sub-paragraph C specifically authorizes flight into severe icing if the airplane is properly equipped.

Yet we already noted the AIM defines severe ice as greater than the de-ice/anti-ice system can handle. Years ago, following several icing accidents, the NTSB recommended in NTSB-SR-81-1 that FAA correct this contradiction, but to date it has not.

The Need for Speed
In 1994 the NTSB made a Safety Recommendation to then-FAA Administrator David Hinson regarding the alarming number of ice-related accidents involving icing-certified aircraft. The accident precipitating the recommendation involved a Beechcraft Duke A60. The problem and recommendations however were and are appropriate to all GA aircraft.

In this mishap, the pilot and five passengers were on their way from the Seattle area to a college football game at Pullman, Wash. They were killed when the aircraft encountered moderate mixed icing conditions during the climb to 17,000 feet.

Radar data showed aircraft climb speed varied from 93 to 118 knots. This was 22 to 47 knots slower than the 140-knot minimum speed for flight in icing conditions published in the 1990 edition of the Beechcraft Twin Engine (Piston) Airplanes Safety Information pamphlet.

While climbing through 13,500 feet at about 100 knots, the airplane made a couple of abrupt altitude deviations. Then from 13,700 feet it entered a steep left spiral. Passing through 12,000 feet the aircrafts pitch attitude was about 70 degrees nose down, with the airspeed increasing rapidly.

There was evidence of an attempted recovery just before the crash, but as the airplane passed through 5,900 feet, its descent rate was 23,000 fpm, with an airspeed of 295 knots. (Redline is 235 knots.) During this graveyard spiral the elevators separated from the horizontal stabilizers and the aircraft hit the ground in a near vertical attitude.

Several pilots who had flown in the area during the same time period reported a rapid ice accumulation in clouds from 3,000 feet to 21,000 feet.

The Board mentioned four accidents involving the Beech Duke Model 60 and A60. No similar mishaps were reported in the Duke B60. The difference, the Board felt, was the POH for the B60 specifically stated a minimum indicated flight speed in known icing conditions of 140 knots indicated.

The Duke 60 and A60 had no such recommendation, only the guideline given in the Beechcraft pamphlet.

At the lower airspeed, ice builds under the wings, aft of the area protected by the boots, where it cannot be seen from the cockpit. This increases stall speed and results in inaccurate warning from the stall warning system. The aircraft stalls well before the warning horn.

The studies showed the accident aircraft (Beech 60 & A60) slowed to around 73 to 78 knots, at which point they entered an uncontrolled descent. This is seven to 12 knots slower than the 85-knot bottom of the green arc (the minimum Normal Operating Speed). In other words the pilots attempted the zoom-climb technique.

Turbines Fall Short, Too
The additional capability of a turbine engine does not insulate pilots from such errors, either. The pilot of an overweight turbo-prop King Air 200 attempted to expedite his climb through a reported layer of severe icing at around 8,000 to 10,000 feet. Radar records showed he, too, attempted to zoom-climb and allowed the airspeed to deteriorate well below the recommended minimum icing airspeed.

Despite the hot wing and tail anti-icing system, the airplane loaded up with ice and crash-landed in a farmers field near Denver. All aboard survived the impact, but the pilot was knocked unconscious. The passengers couldnt locate the emergency exit nor open the main cabin door. A farmer who arrived on the scene was unable to locate the emergency exit. The occupants were asphyxiated by the acrid smoke from the smoldering upholstery.

Two years later the NTSB recommended to the FAA that flight manuals should be amended to include minimum airspeeds and appropriate flight precautions while in icing conditions. It took the FAA four years, but the agency then came out with Advisory Circular 23.1419-1, Certification of Small Airplanes for Flight in Icing Conditions. However the AC was made optional to the manufacturer.

A decade later, following another Duke accident due to in-flight icing, the NTSB issued another recommendation that the FAA amend the POH/AFMs of de-iced airplanes. The recommendation suggested the new information should contain precautionary operational information to help ensure that ice will not accumulate on the under-surface of the wing aft of the area protected by the deicer boots or on other unprotected areas … including specifications of a minimum indicated airspeed that should be maintained during sustained operation in icing conditions.

The main points to remember are that aircraft certified for flight in known ice are approved for flight in moderate conditions only, and even then the systems cannot cope with continuous icing.

The definitions are confusing too. Mixed icing and freezing drizzle and rain all constitute severe icing. No U.S. aircraft is certified for those conditions. And finally remember, when in icing conditions keep your speed up. Your lift depends on it.

Also With This Article
Click here to view “Anti-Ice/De-Ice Systems.”
Click here to view “Icing Certification.”

-by John Lowery

John Lowery is a former Air Force pilot, accident investigator and corporate pilot.


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