“TERRAIN AHEAD, PULL UP,” blared the terrain awareness and warning system (TAWS). This was the first warning to the pilots of a Sukhoi RRJ-95B (Superjet) on the afternoon of May 9, 2012. They were on a demonstration flight that had just left Halim Perdanakusuma International Airport in Jakarta, Indonesia. Six “AVOID TERRAIN” warnings followed. The pilots apparently looked out and saw only clouds. They were not aware of any mountainous terrain and so turned off the TAWS. Minutes later, the airplane crashed into Mount Salak at 6,000 ft. All 45 people on board were killed (ASW, 3/13).
Why did this state-of-the-art airplane with experienced pilots at the controls fly into a mountain? The investigation found that the pilots didn’t have the correct aeronautical charts and that they should have heeded the TAWS aural alerts and warnings. But had they been able to see the terrain features, surely they would have tried to maneuver around them. The crash of the Superjet was yet another case of controlled flight into terrain (CFIT). And it also was just one example of the role weather plays in many of these accidents.
In 2014, none of the major accidents involving commercial jets was classified as CFIT, but in 2013, four of the seven commercial jet major accidents were in that category (ASW, 2/14). From 2009 through 2013, the number of CFIT accidents involving commercial jets steadily increased, although the overall number of accidents decreased.
For commercial turboprop aircraft, eight of the 22 major accidents worldwide in 2013 were CFITs, and these accounted for the majority of fatalities. And the numbers didn’t get much better last year, when seven of the 20 major turboprop accidents were CFITs.
In almost all accidents, a combination of factors and a sequence of events occur. Rarely can you point your finger at one thing and say “that was the cause.” With CFIT accidents, this also is usually true. Human factors, such as a flight crew miscalculation, are common. But weather is a factor in many CFITs. Reductions in visibility are common. Heavy rain, snow, fog and low clouds all are factors. Most CFIT accidents occur during the approach or landing phase of a flight. The risk is compounded when relatively high terrain is in close vicinity to the destination airport. An examination of reports from the following selected CFIT accidents revealed some common factors.
Heavy fog greatly reducing visibility is a recurring scenario in CFITs during landing. On Jan. 29, 2013, SCAT Airlines Flight 760, a Bombardier Challenger CRJ-200, was attempting to land at Almaty International Airport, Kazakhstan. The weather service at the airport reported a runway visual range (RVR) of 200 m (656 ft) and a vertical visibility of 20 m (66 ft) in heavy fog. Lacking the Category IIIB landing capability required in these conditions, the flight was denied landing permission. Shortly after initiating a go-around, the airplane crashed 3 mi (5 km) short of the runway. All 16 passengers and five crewmembers died. Just a month earlier, another CFIT accident had occurred in Kazakhstan, when an Antonov An-72-100 military transport airplane crashed while preparing to land in Shymkent, killing all 27 people on board. Severe icing contributed to a low approach, and poor visibility with fog was noted as the pilot flew the airplane into the side of a ravine.
In a number of cases, the investigation found that a pilot’s actions contributed to the accident. The pilot of Airblue Flight 202, an Airbus A321, was determined to land the aircraft at Benazir Bhutto International Airport in Islamabad, Pakistan, on July 28, 2010. Poor visibility in heavy rain and fog was being reported. The pilot ignored advice from the other flight crewmember and the airport traffic control tower, and only after 21 alerts from the Enhanced Ground Proximity Warning System (EPGWS) did he attempt to reject the landing. The airplane crashed into the Margalla Hills, killing all 157 passengers and crew.
Henan Airlines Flight VD-8387, an Embraer E-190LR passenger jet, was preparing to land at Yichun Lindu Airport, China, on Aug. 24, 2010. Radiation fog was quickly developing as the airplane descended. The tower controller radioed that visibility had dropped below the minimum of 3,600 m (about 2 ¼ mi) and that the flight crew should go around. Although he couldn’t see the runway, the pilot continued, even when the radio altimeter warned him of the height above ground. The airplane crashed 900 m (about 0.5 nm) short of the runway, and an ensuing fire cost the lives of 44 occupants.
On June 20, 2011, RusAir Flight 9605, a Tupolev 134, crashed while attempting to land in Petrozavodsk, Russia, killing 47 of the 52 people on board. Dense fog had lowered visibility below minimum standards and the tower controller had called for a go-around. The flight crew ignored this instruction and continued the approach. The crew continued the descent without visual contact with the runway, and the airplane hit a tree and crashed short of the runway. Only a year before, the crash of a Tupolev 154M on April 10, 2010, in Smolensk, Russia, killed the president of Poland and 95 others; that crash occurred when the crew attempted to land in thick fog. The report said the captain had been warned of the poor visibility and instructed to divert to another airport.
Other accidents in recent years were not categorized as CFITs even though the situations were very similar to those described above. In these cases, the flight crew had been in control but experienced an airplane upset involving restricted visibility. The crash was categorized as loss of control–in flight.
Such was the case in the crash of a Fairchild SA 227-BC Metro III at the Cork Airport in Ireland on Feb. 10, 2011 (ASW, 4/14). On the flight crew’s third attempt to land, the airplane crashed, killing both pilots and four passengers. Two previous attempts to land ended in go-arounds due to poor visibility, which was reported as 450 m (about 0.25 mi) with a broken ceiling at 100 ft. The airplane had descended to 100 ft above ground level when the third go-around was initiated. When the pilot increased power for the go-around, the airplane rolled right and the right wing tip hit the ground.
A similar situation occurred on May 7, 2011, when the crew of Merpati Flight MZ8968, a Xian MA-60, was attempting to land at the Kaimana Airport in Indonesia. As the airplane descended on a visual approach through rain and fog, visibility decreased to 2.0 km (1.2 mi). The airplane had reached 376 ft when the pilots decided to do a go-around. As they increased power, the airplane rolled left and control was lost. The airplane crashed, killing all 25 people aboard.
One factor noted in a number of weather-related CFIT accident reports was spatial disorientation and loss of situational awareness as to height above ground. When visual cues were absent due to fog, clouds or precipitation and pilots did not maintain their instrument scan, they incorrectly gauged attitude, altitude and/or airspeed.
An Afriqiyah Airways Airbus A330-209 crashed while attempting a go-around at Tripoli International Airport in Libya on May 12, 2010 (ASW, 7/13). There were 103 fatalities and one survivor. The airplane descended below the minimum altitude for the approach, and the flight crew could not see the runway environment because of fog. At 228 ft above ground level, the TAWS generated an aural warning. The first officer, who was the pilot flying, applied power and pulled up. Apparently experiencing spatial disorientation, he then made a nose-down sidestick input, and his action caused a steep descent. The captain took the controls, but, because he also likely was spatially disoriented, he moved his sidestick forward, and the airplane struck the ground.
Spatial disorientation may also have been a factor in a March 13, 2014, helicopter crash in Norfolk, England. The crash, which killed four people, occurred in thick fog with visibility of less than 100 m (328 ft). The Agusta Westland AW139 apparently took off without incident and reached an altitude of 125 ft when it pitched nose down and struck the ground 1,380 ft (420 m) from the takeoff point. The U.K. Air Accidents Investigation Branch was continuing its investigation but said that preliminary indications were that there was no technical malfunction of the helicopter.
Other weather conditions also can reduce visibility below requirements. In Kisangani, Democratic Republic of the Congo, on July 8, 2011, a Boeing 727 operated by Hewa Bora Airways crashed while the flight crew was attempting to land. Heavy rain with thunderstorms greatly reduced visibilities. The pilot flying continued the descent although he had no visual contact with the runway. The airplane struck the ground short of the runway and crashed into nearby woods. Eighty-three of the 115 people on board were killed.
In drier climates, strong winds can bring dust storms. Visibilities can drop to near zero in seconds. Such was the case on Aug. 19, 2012, at the airport in Talodi, Sudan. The crew of an Alfa Airlines Antonov AN-26-100 was attempting to land, but visibility was so low due to blowing dust that the tower controller instructed them to go around. While doing the go-around, they crashed into the Nuba Mountains south of Talodi. All 32 people in the airplane died.
Accidents such as the Superjet crash described above represent a rare subset of CFITs — those that occur while an aircraft is not landing or on an approach. These accidents often occur in mountainous terrain, which often is partly or completely obscured by clouds. Whenever air is lifted, it cools. When the air cools to its dew point, condensation occurs and clouds form. Any type of wind that blows upslope can produce this effect. Thus, the windward side of mountains is especially prone to clouds. The level at which clouds will begin to form depends to a large extent on the moisture content of the air; the more moisture, the lower the cloud base. Even without these orographic effects, low clouds from other lifting sources pose problems because the mountaintops often extend into the low cloud deck.
In addition to the Superjet accident, numerous other accidents have involved an aircraft striking high mountainous terrain. Nepal Airlines Flight 183, a de Havilland DHC-6-300 Twin Otter, crashed into terrain at 7,000 ft near Dhikura, Nepal, on Feb. 16, 2014. All 18 people aboard were killed. Noted weather conditions included rain, fog and mountain snow.
Pamir Airways Flight 112 impacted a mountainside at 13,500 ft in Afghanistan, killing all 39 passengers and five flight crewmembers on May 17, 2010. Officially ruled a “weather-related accident,” the Antonov An-24 encountered strong winds and obstructed visibility due to snow and clouds.
Besides the limited-visibility cases, there were other CFIT accidents in which weather was a major factor. For example, strong winds and turbulence were cited in the accident involving Lao Airlines Flight 301, a scheduled passenger flight to Pakse International Airport in Laos on Oct. 16, 2013, which crashed into the Mekong River, killing all 49 aboard. The ATR 72-600 was attempting its second approach when the accident occurred. The first landing attempt was discontinued due to strong winds and heavy rain. A dissipating typhoon was affecting the region.
Thirty-two people died when a Bombardier CRJ-100ER crashed while attempting to land at the Kinshasa N’Djili Airport in Democratic Republic of the Congo, on April 4, 2011. A thunderstorm described as severe was occurring at the airport. A wind shear warning was noted in the cockpit as the pilots were performing a go-around. Investigators believe the airplane encountered a strong downdraft or microburst that forced it to the ground.
On April 20, 2012, a Boeing 737-200 crashed while attempting to land at Benazir Bhutto International Airport, Islamabad, Pakistan. All 127 onboard perished. On final approach, the aircraft encountered a thunderstorm. Wind shear was noted with gusts to 34 kt. An apparent downburst drove the plane down as the TAWS sounded an alert. The pilot was unable to pull up in time to avoid striking the ground.
In the U.S. general aviation community, fatal CFIT accidents have been declining. According to an FAA Fact Sheet released in early 2014, “Over the past three years, fatal accidents from … CFIT have been reduced by more than 50 percent compared to the previous three years.”1 However, it was pointed out that for the period 2001–2011, CFIT accidents were the second leading cause of fatal general aviation accidents (loss of control–in flight was first). The FAA also noted that “25 percent of all weather-related accidents are fatal.” Pilots’ failure to recognize deteriorating weather conditions was singled out as a frequent cause. For example, on July 27, 2013, a Robinson R66 helicopter crashed near Noxen, Pennsylvania, killing all five aboard. The pilot had radioed, “We’re inadvertent IMC [instrument meteorological conditions]. Having trouble maintaining altitude.” Thunderstorms and heavy fog were in the area.
What conclusions can we draw? Certainly, weather is a factor in many CFIT accidents. Reduced visibility due to dense fog or clouds is the most common problem. In almost all instances, pilots went ahead with approaches or landings knowing the weather conditions. Certainly equipment like TAWS helps, but even with these advanced systems, pilots still can make critical misjudgments.
Edward Brotak, Ph.D., retired in 2007 after 25 years as a professor and program director in the Department of Atmospheric Sciences at the University of North Carolina, Asheville.
- FAA. Fact Sheet – General Aviation Safety. Jan. 27, 2014.
Featured image: © Dingalt | Dreamstime
Sukhoi: Katsuhiko Tokunaga | Wikimedia CC- BY-SA 2.0
Mount Salak: Ehamberg | Wikimedia CC-BY-SA 3.0