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The need to understand weather even when there appears to be no weather is crucial for aviators. The apparent lack of weather can be misleading and give a false sense of security. As aviators, we must ensure that we obtain and understand the weather that will impact every flight we undertake. Possibly as a result of what appeared to be good weather and a lack of complete and accurate weather data from ATC, the pilots of Continental Airlines Flight 1404 out of Denver, Colorado on December 20th, 2008 were surprised by the crosswind gusts that resulted from apparent mountain wave which caused the aircraft to depart from the left side of the runway during the takeoff roll.
Understanding the bigger picture of what is going on around us as it pertains to weather can help us make better go/no-go decisions.
Aviators must develop a thorough understanding of weather. It is crucial to the safety of every flight. When we think of weather, we tend to think of bad weather or simply weather we can see.
There is more to weather than simply clouds and precipitation. Understanding what causes weather can help us be safer pilots by not taking unnecessary risks. We can get lulled into a false sense of security when we don’t see weather. We have all experienced windy days when there were no other weather conditions present. Developing an understanding of what causes weather enables us to make decisions about conducting flight operations. As pilots, we are the final decision authority about the safety of any flight we are in command of.
We must use this knowledge for our flights to predict if the weather is changing.
The pilots of Continental Airlines Flight 1404, a Boeing 737 out of Denver, Colorado on December 20th, 2008, were experienced pilots. The captain had 13,100 total flight hours with 6,300 in the 737 and the first officer had 8,000 total flight hours with 1,500 in the 737. (NTSB, 2008) They were both completely qualified for the flight, and yet the severity of the wind conditions surprised them. There were no issues with the aircraft or the crew physically or otherwise. The result of this flight was due to mountain wave that resulted in significant changes in the wind strength, which was a crosswind to runway 34R. When the pilots received their weather information, they were not advised of the mountain wave conditions. (NTSB, 2008) However, as pilots we are taught about this weather phenomena and must understand its potential effects. The flight was being conducted out of Denver, Colorado, which is in mountainous terrain. This should be an obvious indication that mountain wave is a potential danger for operating at this airport.
The pilots were informed by ATIS that there was an 11-knot wind prior to push back from the terminal. After taxiing to runway 34R and provided final takeoff clearance, ATC indicated a 27-knot wind from 270°. The max crosswind for Continental Airlines 737 aircraft was published as 33-knots. (NTSB, 2008) With the winds at such a significant difference from the time of pushback to takeoff, it might have been an indication that the mountain wave was increasing in strength. The pilots might have considered requesting a delay to allow the mountain wave to pass and the winds to decrease. This would have prevented them from having to attempt to deal with the winds that maxed out around 45-knots during their takeoff roll.
As a result of the strong wind from the mountain wave, the pilot applied the rudder as the nose of the airplane began to swerve left. Through the rudder applications made by the pilot the aircraft was moving back and forth across the runway with the application and release of the rudder. The pilot attempted to also use the tiller to control the nose wheel. (NTSB, 2008) It was stated that the pilots attention should have been to reapply the rudder, but when this did not happen, the aircraft departed runway 34R to the left. Fortunately, there were no fatalities as a result of this accident.
The need to understand the weather is critical. These pilots were surely trained in understanding weather and were experienced in the application of weather, but it appears that the effects of mountain wave caught them unprepared. Had the pilot’s thoughts been on the potential mountain wave, it is possible the re-application of the rudder could have kept the aircraft on the centerline and they could have completed the takeoff or rejected the takeoff and stopped on the runway. (NTSB, 2008) Had ATC told the pilots that mountain wave was an issue, it would have certainly triggered their minds to prepare for the increases and decreases in wind strength.
Understanding weather means obtaining as much information as possible to create an accurate understanding of what is happening around us. As discussed in chapter 16 of our text, Aviation Weather, there are many resources available to us. Some of these resources include Terminal Aerodrome Forecasts, Area Forecasts, METARs, and more. (Lester, 2013) The information we gather must include not only the weather resources available to us, but also our operating environment. We must consider the airports we are flying out of and into. We must think about our planned altitude(s) and what terrain features our flight path will take us over. These are all crucial to understanding what weather might impact our flight.
When our operating environment includes mountains or operating close enough to them that the effects of mountain wave can impact our flight, then we must keep this in mind. As I was flying in a Cessna 172 roughly 45 minutes east of the Blue Ridge mountains, I was close enough to feel the effects of mountain wave at approximately 3,000 feet MSL. The surges of wind would cause the aircraft to climb and descend. Without understanding what was going on, I was challenged to maintain my assigned altitude. It took constant adjustment of the throttle and application of the yoke to pitch up or down to counteract the mountain wave. At the time, I was not thoroughly aware of mountain wave. My instructor was however and was able to provide a real-world experience that I will never forget.
Mountain waves are formed by the movement of stable air over mountainous terrain. The length of the waves can vary, but once established usually don’t change until the passage of the air over the mountain is complete. (Lester, 2013) Using this information, careful observation of the changes in the winds can help develop a timing pattern of when the waves begin and end. This information would have been helpful to the pilots in our case study.
In our case study, the Denver airport is in the mountains, and therefore susceptible to mountain wave. The pilots should have been anticipating the mountain wave based on the information available to them and what they were being provided. Had they opted to wait as little as a minute or two, the strongest winds would have subsided, and the takeoff would have been uneventful. The application of our knowledge of the weather usually comes down to deciding to delay our flight until the weather is acceptable. Understanding weather includes developing the knowledge of what causes the formation of the weather we encounter. We must then apply this knowledge in a manner that ensures the safety of every flight we conduct. If we merely understand weather and don’t apply it, we can find ourselves in a situation we are unprepared to deal with. An example would be that we understand what icing is, but if we don’t apply our knowledge and ensure we are prepared, we might fly into icing and not have de-icing equipment available. Likewise, if we apply knowledge that is not complete, the level of safety is limited to our ability operate only in the weather we understand.
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