| As the northern hemisphere slips into the cold seasons, cautious pilots will review the ice protection information located in their airplane’s documentation (AFM, POH, etc.) and their company’s winter operating procedures. Ice destroys lift and chokes off power. Ice takes down large aircraft and small. Ice is insidious. Small amounts of barely visible ice on a modern high-performance wing can glue your airplane to the ground and send you rolling off the end of the departure runway at takeoff velocity with no options and lousy prospects. Perhaps worse — it can launch your aircraft into the air until it runs out of ground effect and stalls.This month we’ll look at the loss of a Ryan International DC-9 (Series 10) and its two pilots. The aircraft crashed and burned in an attempt to take off from Cleveland Hopkins International Airport (CLE) on Feb. 17, 1991. In the view of the NTSB, this accident should be studied by pilots who want to understand how small quantities of ice (and a lack of information) can so impact the effectiveness of a modern airfoil that the destruction of a transport jet can result.
After an extended investigation, the Safety Board determined that the probable cause of the accident was “the failure of the flight crew to detect and remove ice contamination on the airplane’s wings, which was largely a result of a lack of appropriate response by the FAA, Douglas Aircraft Co., and Ryan International Airlines to the known critical effect that a minute amount of contamination has on the stall characteristics of the DC-9 Series 10 airplane. The ice contamination led to wing stall and loss of control during the attempted takeoff.” It should be noted that all parties ultimately responded positively to the NTSB’s recommendations, but takeoff accidents continue to occur.
The DC-9 is hardly alone among turbine aircraft to be susceptible to lift deterioration resulting from contamination by small amounts of ice. Aircraft without leading edge devices seem to be the most vulnerable. Here’s a list of turbine aircraft accident reports with similar probable cause findings:
Canadair CL-600-2A12 — Crash During Takeoff in Icing Conditions, Montrose, Colo., Nov. 28, 2004. Aircraft Accident Report NTSB/AAB-06/03.
Canadair CL-604 — Epps Air Service Crash During Takeoff, Birmingham International Airport, Birmingham, U.K., Jan. 4, 2002. Air Accidents Branch (AAIB), Department of Transport, U.K. Aircraft Accident Report 5/2004 (EW/C2002/1/2).
Cessna Caravan — NTSB recommendation letter issued as a result of 26 Cessna 206 icing-related incidents and accidents.
Fokker F-28 — Takeoff Stall in Icing Conditions, USAir Flight 405, LaGuardia Airport, Flushing, N.Y., March 22, 1992.
MD DC-9-14 — Takeoff in Icing Conditions, Continental Airlines, Stapleton International Airport, Denver, Colo. Nov. 15, 1987.
The Ryan InvestigationRyan 590, a contract mail flight, had originated at Greater Buffalo, N.Y., International Airport (BUF) at 2255 on Saturday, Feb. 16, with a scheduled stop at CLE before its final destination at Indianapolis International Airport (IND).
Weather in the area was typical for northern winters — messy. At 2220, the NWS surface weather map showed a low-pressure area centered over western Ontario, north of Lake Huron. A cold front extended southwestward from the low through the extreme northwestern portion of Lake Huron and northern Lake Michigan. The front turned from there west-southwestward from the low through central Wisconsin and along the Iowa-Minnesota border. The map also showed a warm front extending south-southeastward from the extreme northeastern portion of Iowa into central Illinois, then turning southward into the extreme southeastern portion of Missouri.
The following surface observations were taken by the NWS at Cleveland:
Time 2350
Type record special
Ceiling indefinite, 1,500 ft. obscured
Visibility 1 mi. variable
Weather light snow
Temperature 23°F
Dew point 19°F
Wind 220 deg. at 14 kt.
Altimeter 29.91 in.
Remarks Runway 5R visual range 6,000 ft. plus, visibility 0.75 mi. variable 1.5 mi.
As Ryan 590 let down into the Cleveland area, an approach controller passed on information from combined PIREPS that stated: “two pilot reports moderate rime icing reported 7,000 ft. on to the surface during the descent that was by a 727, and also moderate chop turbulence from 4,000 ft. on to the surface.”
The Ryan crew acknowledged receiving this information as they executed an ILS approach to Runway 23L at CLE. There is evidence that they had activated the aircraft’s anti-ice systems.
The Ryan flight touched down at 2344. The pilots taxied to the mail ramp where some of the mail from BUF was unloaded and additional mail destined for IND was put aboard. The pilots remained in the cockpit during this transfer. At the time, Ryan procedures did not require DC-9 crews to accomplish a walk-around inspection at intermediate stops. Dry blowing snow fell throughout the 35 min. Ryan 590 was on the ground.
Investigators said neither Ryan 590 nor any other flight that took off from Cleveland during the evening or early morning hours of February 16-17 requested or received deicing service. (Ten minutes after the accident, the temperature at CLE was 23°F, and the dew point was 20°F).
Cleveland tower issued the departure clearance to Ryan 590 at 0006:38 (Sunday). At 0009:18, the flight crew asked for taxi clearance from “south cargo.” The controller issued the clearance and informed the flight crew that the last reported braking action, which the flight crew had described as “fair” was when Ryan 590 arrived. The crew taxied for takeoff on Runway 23L and, at 0018:17, was cleared for takeoff to “fly runway heading.”
Later some witnesses described seeing the airplane lift off from the runway and climb 50 ft. to 100 ft. agl before it rolled to the right, then to the left past the 90-deg. position, and crashed. Other witnesses described the first unusual movement as a slight roll to the left, followed by a substantial roll to the right, with an increase in pitch attitude, and a more severe roll to the left before impact. Some witnesses saw flames coming from the left engine.
The CVR tape revealed that the captain made the following callouts during the takeoff sequence: “Vee one,” at 0018:44; “Rotate,” at 0018:45; “Vee two,” at 0018:48; “Plus 10,” at 0018:49; and “Positive rate,” at 0018:50.2 The captain then warned the first officer three times in quick succession to “Watch out,” beginning at 0018:51 and ending about one second later. At 0018:52, immediately after the last call to “Watch out,” the CVR recorded sounds similar to engine compressor surges and, at 0018:53, the sounds of a stick shaker. The sound of the first impact occurred at 0018:57.
The airplane’s left wing had struck the snow-covered grass on the right side of the takeoff runway. It left a 1,600-ft. path of wreckage off the right side of the runway and came to rest inverted on the runway about 6,500 ft. from the threshold. Impact marks started on the right edge about 5,078 ft. from the beginning of the runway, and continued to the point at which the fuselage came to rest. Rescue crews extinguished a ground fire and found both fatally injured pilots in the cockpit. There were no passengers on board.
Inspection of the runway uncovered a faint scar, about 100-ft. long, located approximately 3,440 ft. from the beginning of the runway. Examination of the bottom of the tail of the airplane revealed a worn or flattened area on the hard metal tie-down eye.
Investigators’ AnalysisAfter determining that there were no preexisting medical problems with the pilots or mechanical problems with the airplane, investigators turned to weather and the airplane’s performance in icing conditions. The fireball was probably a result of engine surges as the aircraft stalled.) The 14-kt. winds were not considered a factor.
“The abrupt decrease in the airplane’s normal acceleration, the entry of the airplane into a steep roll attitude, the sounding of the stall warning stick shaker, and the occurrence of engine compressor surges at an airspeed 27 kt. above the theoretical stall speed for the given conditions clearly indicate that the aerodynamic lift-producing capability of the wings was degraded,” said investigators.
They explored possible causes for this loss of aerodynamic efficiency, such as an improper takeoff configuration, extension of wing spoilers and contamination or roughness of the airfoil surface. Because the evidence did not support either an improper takeoff configuration or an extension of wing spoilers, investigators focused on the possibility that some amount of ice or frozen snow was present on the wing leading edge or upper surface and that this contamination affected the airplane’s flight characteristics.
As it turns out, the surface conditions at CLE were not necessarily conducive to accumulations of airframe ice because of the relatively low ambient temperature and the relatively dry snow. “However, the airplane did fly through moderate rime icing during its descent for landing at Cleveland,” said the Board.
Investigators said the flight crew had received ample weather information, including a PIREP, about icing conditions around CLE during the approach, so they should have selected both wing and engine icing protection during the descent for landing. And there is reference on the CVR to use of these systems while the airplane was on the ground at CLE. The Safety Board found no evidence to suggest that the anti-ice system was inoperative during arrival at CLE, nor did it find evidence that the anti-icing system had caused runoff from the heated surfaces that refroze on the upper wing surface. (On the other hand, the possibility that runoff moisture refroze could not be ruled out.)
The most likely explanation for the formation of ice on the wing surface is that the flight crew used the wing anti-ice system during the approach and that the falling dry snow melted and refroze while the airplane was on the ramp at Cleveland. The Safety Board believes that this scenario is possible because the wing would be “hot” upon touchdown (when the air/ground relay deactivates the anti-ice system automatically) and the blowing dry snow can melt on the wing and refreeze, as the wing temperature cools to below freezing. The Board noted that it had investigated four previous DC-9 Series 10 incidents in which the airplanes had flight control difficulties during takeoff with wing ice contamination.
Performance engineers determined that the aircraft’s acceleration and takeoff were normal until the airplane was rotated and lifted off the ground. The roll at 100 ft. agl and rapid descent after liftoff were the result of an aerodynamic stall, they said. As in the previous (DC-9) accidents, the airplane was able to lift off and climb initially because of the influence of ground effect on the aerodynamic characteristics of the wing. (When an airplane is close to the ground plane, the direction of airflow over the wing is altered. The result is that the wing will produce more lift at the same airspeed and AOA than it will when the airplane is in free air. This enhanced aerodynamic performance diminishes as the airplane climbs and becomes almost negligible at a height equal to the airplane’s wingspan, a distance of 87 ft. for the DC-9-15.)
Generally, an airplane’s rotation speed is selected so that, with a normal rate of rotation, the airplane will lift off at a speed that offers a safe margin above the stall speed. In this case, the first officer rotated the airplane at the proper airspeed (132 kt.) to ensure this stall margin.
The aerodynamic performance degradation notwithstanding, the Ryan airplane reached a combination of airspeed and AOA at which a vertical lift was developed that exceeded the airplane’s gross weight. However, the acceleration after liftoff was sluggish compared with typical takeoffs and acceleration stopped altogether after only two seconds. That’s when the captain called, “Watch out” and, one second later, the airplane’s heading deviated abruptly to the left and the engine compressor surges began. The Safety Board believes that this combination of events is consistent with an abrupt and unsymmetrical aerodynamic stall of the wings as the airplane reached a height where it lost the aerodynamic performance advantage of ground effect.
The stall occurred at about 150 kt. The engineers determined that the lift coefficient for the wing of the accident airplane was nearly 30% less than the theoretical lift coefficient for a DC-9 Series 10 wing.
“This degradation in aerodynamic performance is consistent with the performance decrement caused by minute amounts of contamination as cited by the manufacturer in several technical articles. According to the manufacturer, a wing upper surface contamination that is only 0.014-in. thick, about equal to the roughness of 80-grade sandpaper, can produce a 25% loss of wing lift. Therefore, the Safety Board concluded that the decrement in the aerodynamic lift-producing ability of the accident airplane was caused by an ice or snow accumulation on the wing that may have been less than 0.02-in. thick and barely perceptible from visual observation.”
Ryan Pilot GuidanceMuch information on the subject of airfoil contamination had been developed by McDonnell Douglas dating as far back as January 1969. “However,” noted the Safety Board, “it is unlikely that the Douglas publications or All Operator Letters were sent to Ryan because, at the time of distribution, the company did not operate Douglas aircraft.” Consequently, no specific information regarding the DC-9 icing history or special precautions relating to ground deicing was given to Ryan line pilots, who were ultimately responsible for the safe operation of the aircraft.
The DC-9 Operations Manuals were basically developed by Ryan from the airplanes’ previous owner’s Operations Manuals, and certain purported Ryan practices were not incorporated into them. The requirement to conduct an exterior inspection of the airplane at intermediate stops was one of those practices not incorporated. In fact, the preflight inspection requirement in the Ryan DC-9 manual clearly indicated that exterior inspections were required only on originating flights or after the airplane had been left unattended. In contrast, the Ryan Operations Manual for the B-727 specified that an exterior inspection was required before each flight and assigned the conduct of the inspection to the second officer.
Following the accident, when Ryan discovered the DC-9 accident history and icing data, as well as the oversight regarding walk-around inspections at intermediate stops, the company published Operations Bulletin 91.3, which includes the following guidance: “When weather conditions exist that may cause wing contamination, the leading edge and upper surface shall be inspected. A ladder has been provided for this purpose. NO AIRCRAFT may depart any station with any wing or tail contamination. If in doubt, DE-ICE!”
Unfortunately, the accident crew did not detect ice on the wings. “After failing to detect and remove the accumulation of ice from the wing, there were no actions that the crew could have been reasonably expected to take that would have prevented this accident. The first officer followed the normal and prescribed procedures for the takeoff; that is, the rotation speed was that specified for the airplane’s weight and the rotation rate was normal.
When the airplane became airborne with a minimum stall speed margin, the stall was inevitable as the aerodynamic advantage of ground effect diminished. Further, the stall was most likely more sudden and severe than would have occurred with an uncontaminated wing because a stall can progress from the wingtips inward. This causes the airplane to pitch nose up with a loss of roll control. The abrupt roll, occurring as one wing stalled before the other, was not controllable within the altitude available.”
Lessons LearnedEach year the NTSB urges pilots to review their performance manuals and SOPs regarding icing. The best quick reference on this subject is the Safety Board’s “Aircraft Ground Icing Safety Alert.” We’ve made note of this Safety Alert in previous columns. If you’ve already read what follows, pass it on to a pilot who may have failed to get the word. After all, the failure to get this information is what killed the Ryan crew.
The problem:
Fine particles of frost or ice, the size of a grain of table salt and distributed as sparsely as one per square centimeter over an airplane wing’s upper surface, can destroy enough lift to prevent an airplane from taking off.
Almost virtually imperceptible amounts of ice on an aircraft wing’s upper surface during takeoff can result in significant performance degradation.
Small, almost visually imperceptible amounts of ice distributed on an airplane’s wing upper surface cause the same aerodynamic penalties as much larger (and more visible) ice accumulations.
Small patches of ice or frost can result in localized, asymmetrical stalls on the wing, which can produce roll control problems during liftoff.
It is nearly impossible to determine by observation whether a wing is wet or has a thin film of ice. A very thin film of ice or frost will degrade the aerodynamic performance of any airplane.
Ice accumulation on the wing upper surface may be very difficult to detect from the cockpit, cabin, or front and back of the wing because it is clear/white.
Accident history shows that nonslatted, turbojet, transport-category airplanes have been involved in a disproportionate number of takeoff accidents where undetected upper wing ice contamination has been cited as the probable cause or sole contributing factor.
Most pilots understand that visible ice contamination on a wing can cause severe aerodynamic and control penalties, but it is apparent that many pilots do not recognize that minute amounts of ice adhering to a wing can result in similar penalties.
Despite evidence to the contrary, these beliefs may still exist because many pilots have seen their aircraft operate with large amounts of ice adhering to the leading edges (including the dramatic double horn accretion) and consider a thin layer of ice or frost on the wing upper surface to be more benign.
Pilot Knowledge and ActionsPilots should be aware that no amount of snow, ice or frost accumulation on the wing upper surface should be considered safe for takeoff. It is critically important to ensure, by any means necessary, that the upper wing surface is clear of contamination before takeoff.
The NTSB believes strongly that the only way to ensure that the wing is free from critical contamination is to touch it.
With a careful and thorough preflight inspection, including tactile inspections and proper and liberal use of deicing processes and techniques, airplanes can be operated safely in spite of the adversities encountered during winter months.
Pilots should be aware that even with the wing inspection light, the observation of a wing from a 30- to 40-ft. distance, through a window that was probably wet from precipitation, does not constitute a careful examination.
Pilots may observe what they perceive to be an insignificant amount of ice on the airplane’s surface and be unaware that they may still be at risk because of reduced stall margins resulting from icing-related degraded airplane performance.
Depending on the airplane’s design (size, high wing, low wing, etc.) and the environmental and lighting conditions (wet wings, dark night, dim lights, etc.) it may be difficult for a pilot to see frost, snow and rime ice on the upper wing surface from the ground or through the cockpit or other windows.
Frost, snow and rime ice may be very difficult to detect on a white upper wing surface and clear ice can be difficult to detect on an upper wing surface of any color.
Many pilots may believe that if they have sufficient engine power available, they can simply “power through” any performance degradation that might result from almost imperceptible amounts of upper wing surface ice accumulation. However, engine power will not prevent a stall and loss of control at liftoff, where the highest angles of attack are normally achieved.
Some pilots believe that if they cannot see ice or frost on the wing from a distance, or maybe through a cockpit or cabin window, it must not be there — or if it is there and they cannot see it under those circumstances, then the accumulation must be too minute to be of any consequence. The consequences of those erroneous beliefs can be severe. BCA
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