A Bad Time For Circling: Picking up airframe ice on the approach presents handling challenges at a critical time, especially if we don't realize it.
One of the more insidious times we pick up airframe icing is when descending for the destination airport. We can be in good VMC during cruise, but then encounter ice in the descent and have to haul it all the way to the runway. And thanks to the increased workload of an IFR arrival and approach in solid IMC, we may not even know we've picked up ice if we don't specifically check. Even if we are aware we're carrying it, we can still screw things up. That's mainly because we're trying to slow down enough to land, and any appreciable ice accumulation likely will have added drag and changed the airplane's aerodynamics.
Changes to the airplane's aerodynamics usually will include lowering its critical angle of attack, beyond which a stall results. The most common adverse outcome might be "only" a hard landing. The worst outcome is, well, the worst. And if the landing approach includes a circle-to-land maneuver, we're setting ourselves up for one of winter flying's classic traps.
On January 26, 2016, at about 1754 Eastern time, a Cirrus Design Corporation SR22T impacted terrain as it turned final to land on Runway 25 at the Greene County-Lewis A. Jackson Regional Airport in Xenia, Ohio. The solo pilot died and the airplane was substantially damaged. Visual conditions prevailed in the area during the approach. The flight had been airborne for about an hour and was reaching its destination on an IFR flight plan.
The pilot was cleared from 9000 feet msl down to 3000, and then was cleared for the RNAV (GPS) Runway 7 approach procedure into Greene County. Approach control issued pilot reports for icing. About 5.8 miles from the airport, the pilot cancelled his IFR clearance and continued inbound VFR. His recorded altitude at that time was 2700 feet msl; field elevation is 946 feet msl. An airport employee noted the local weather observation advertised a 1700-foot agl overcast ceiling with wind from 240 degrees at nine knots. Winds included variable gusts from 240 to 330 degrees, to 14 knots.
Witnesses observed the airplane on what appeared to be a downwind to Runway 25 and saw the airplane flying low. It then entered a steep left bank and nose-dived toward the ground into the trees.
The airplane came to rest about 300 feet short of the Runway 25 threshold, oriented on a 284-degree heading. The airplane impacted in a nose-down attitude. All of its major components were confirmed to be in the vicinity.
A ground scar was present immediately forward of the right wing, which remained attached to the fuselage. Dirt adhered to the upper wing surfaces consistent with the wing skin being coated with TKS ice protection fluid during flight. The left wing was lying flat on the ground and exhibited upward crushing and buckling.
The airplane's remote data module (RDM) is an impact- and fire-hardened recorder mounted in the tail. The unit records approximately 105 flight-data parameters at a rate of once a second. Some of the datapoints retrieved from the accident airplane's RDM included:
* The TKS anti-ice switch was turned on about seven minutes and 30 seconds prior to the accident for one minute and 50 seconds. It was then turned off and remained off for the remaining five minutes and 40 seconds until the accident.
* The airplane's flaps were deployed to the 50-percent position two minutes and 50 seconds before the accident.
* Just before the data ended, pitch and bank increased and the stall warning activated. In the last three seconds, the airplane's bank angle was 48 to 50 degrees. Its indicated airspeed was between 87 and 90 knots, and vertical speed increased from a 240-fpm descent to 1056 fpm.
The Pilot's Operating Handbook showed that at 60 degrees of bank with half flaps, the airplane's stall speed is 95 KIAS.
At 1732, the observed temperature at Greene County was 0 degrees C, with a dew point of -2 C. An upper air sounding 18 miles from Greene County taken an hour after the accident "showed high icing potential within the cloud layers above the surface." Infrared satellite imagery from 1730 to 1815 showed cloud top temperatures of -12 C, corresponding to cloud tops around 12,000 feet and "indicating the likelihood of moderate or greater icing along the airplane's route of flight...." Airmets called for moderate icing conditions below 12,000 feet msl and an Indianapolis Center statement warned of occasional moderate rime and mixed icing between 2000 and 5000 feet msl.
Pilot reports received two hours prior to the accident and one hour after the accident indicated light and moderate rime to moderate mixed icing in the clouds below FL200 near Greene County. There was no record of the pilot contacting Lockheed Martin Flight Service or DUATS for weather or Notams.
The NTSB determined the probable cause(s) of this accident to include: "The pilot's failure to maintain adequate airspeed while turning from the base leg to final, which resulted in the wing's critical angle-of-attack being exceeded and a subsequent aerodynamic stall."
The NTSB provided additional comments: "It is possible that, during the approach, ice accumulated on the airplane, which may have increased the airplane's stall speed. However, regardless of whether or not structural ice was present, during the turn to final, the pilot allowed the airspeed to decrease below the airplane's published stall speed. As a result, the wing's critical angle-of-attack was exceeded, and the airplane entered an aerodynamic stall and departed controlled flight."
The punchline in all this? In addition to the RNAV (GPS) approach to Runway 7 at Greene County, there's also a similar RNAV (GPS) approach to Runway 25. It actually features a 50-foot lower decision altitude. If the accident pilot had opted for the straight-in procedure, no steep turns close to the ground would have been necessary, and we'd likely be talking about a different accident this month.
BY JOSEPH E. (JEB) BURNSIDE
EFFECTS OF AIRFRAME ICE ON AIRCRAFT HANDLING
According to the FAA's Instrument Flying Handbook (FAA-H-8083-15B), "The effects of ice on aircraft are cumulative--thrust is reduced, drag increases, lift lessens, and weight increases. The results are an increase in stall speed and a deterioration of aircraft performance. In extreme cases, two to three inches of ice can form on the leading edge of the airfoil in less than 5 minutes. It takes only 1/2 inch of ice to reduce the lifting power of some aircraft by 50 percent and increases the frictional drag by an equal percentage.
"A pilot can expect icing when flying in visible precipitation, such as rain or cloud droplets, and the temperature is between +02 and -10[degrees] Celsius. When icing is detected, a pilot should do one of two things, particularly if the aircraft is not equipped with deicing equipment: leave the area of precipitation or go to an altitude where the temperature is above freezing. This 'warmer' altitude may not always be a lower altitude. Proper preflight action includes obtaining information on the freezing level and the above-freezing levels in precipitation areas."
Trace Ice becomes perceptible. Rate of accumulation is slightly greater than sublimation. Anti-icing/deicing equipment is not utilized unless encountered for an extended period of time (over 1 hour). Light The rate of accumulation may create a problem if flight is prolonged in this environment (over 1 hour). Occasional use of anti-icing/deicing equipment removes/prevents accumulation. It does not present a problem if anti-icing/deicing equipment is used. Moderate The rate of accumulation is such that even short encounters become potentially hazardous and use of anti-icing/deicing equipment or flight diversion is necessary. Severe The rate of accumulation is such that anti-icing/deicing equipment fails to reduce or control the hazard. Immediate flight diversion is necessary.
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|Title Annotation:||ACCIDENT PROBE|
|Author:||Burnside, Joseph E.|
|Article Type:||Case study|
|Date:||Jan 1, 2018|
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