Aircraft incident has always been tragic and scary. Despite the fact that aviation is one of the fastest developing industry there are still many uncertainties regarding its safety. As the title suggests there might be hidden danger in an aircraft you are flying and even pilots and flight attendants might have no clue about it. It is hard to believe it but this has happened in the past and here is an incident review reflecting this topic.

If you are an aerospace aficionado, also check out this post: Aerodynamic Forces: How does an Aircraft Fly?

United Air Flight 585 Incident

On March 3, 1991, United Airlines Flight 585 was scheduled to fly from Denver to Colorado Springs, Colorado carrying 20 passengers and 5 crew members. The flight was operated by a Boeing 737-291 registered as N999UA. The aircraft was scheduled to land at 9:46 AM and at 9:37 AM, the aircraft was cleared for a visual approach to runway 35. Then all of a sudden the aircraft entered a spiral dive. The crew tried to mitigate the dive by selecting 15-degree flaps and an increase in thrust but unfortunately this wasn’t enough. The aircraft crashed into Widefield Park, less than four miles from the runway threshold, at a speed of 245 mph. All 25 people onboard were killed.

United Airlines Flight 585 crash site

The NTSB (National Transportation Safety Board) began an investigation which lasted 1 year and 9 months. They considered the possibility of a malfunction of the rudder power control unit servo which might cause the rudder to reverse any inputs. What’s more, they also considered the effect of powerful rotor winds from the nearby Rocky Mountains. However, the Flight Data Recorder only recorded the aircraft’s heading, G loads and microphone keying and didn’t record rudder, aileron or spoiler deflection data, which could have helped the NTSB’s investigation. Eventually, because of the lack of evidence the NTSB couldn’t explain the reasons behind the accident

US Air Flight 427 Incident

Three years later on September 8, 1994 US Air Flight 427 was preparing to land at Pittsburgh International Airport after a flight from Chicago O’Hare International Airport. After the stopover there, the plane is scheduled to continue on to West Palm Beach International Airport, Florida. The aircraft was a Boeing 737-387 registered as N513AU. There were total 132 people onboard including 127 passengers and 5 crew members. After negotiation with Air Traffic Controller, the flight was getting ready to land on runway 28R. But suddenly the aircraft behaved erratically and entered a dive. The plane crashes into Hopewell Township, Beaver Country, near Aliquippa, Pennsylvania and unfortunately all 132 were killed.

US Air Flight 427 crash site- Aircraft incident

The aftermath of the aircraft incident was horrible as the incident site was declared a bio-hazard. The NTSB launched what would be the longest investigation in its history. Though NTSB investigated rudder problems being a potential issue, it took more than 4 and a half years and another incident on Eastwind Airlines Flight 517 until conclusion was reached.

Eastwind Airlines Flight 517

On June 9, 1996 Eastwind Airlines Flight 517 with 53 people onboard was preparing to land at Richmond International Airport, Virginia, after a flight from Trenton-Mercer Airport, New Jersey. The aircraft is Boeing 737-2H5 registered as N221US. While descending through 5000 feet, the plane’s rudder suddenly deflected to right then automatically returns back to normal and the Captain used the ailerons to compensate.

Again passing through 4000 feet, the plane abruptly yawed right as the rudder jolted right. The captain applied full left rudder but it was stiff and so he applied left aileron and also increased thrust to the right engine. It worked but he was still fighting with the plane and finally completes the emergency checklists which involved disabling the yaw damper. This helped him gain the full control of the plane and the plane landed safely. All of them survived while a flight attendant was injured.

Eastwind Airlines Flight 517

NTSB Investigation

After that, investigations were made on the aircraft. During the investigation, the NTSB found that prior to the June 9 incident, flight crews had reported a series of rudder-related events on the incident aircraft, including abnormal “bumps” on the rudder pedals and uncommanded movement of the rudder Using the evidences from other flights, pilot interviews and Flight Recorder information, NTSB was able to determine the probable cause.

NTSB Investigation Findings

After conducting several on-ground and in-flight tests flaws were discovered in the 737’s Rudder Power Control Unit(PCU). They discovered that injecting a cold PCU with hot hydraulic fluid caused the PCU servo’s secondary slide to become jammed against its housing. This resulted in rudder movements opposing pilot’s command and even uncommanded movements. The higher speed was what saved the Eastwind Flight. Greater airflow over the wings made ailerons have a larger effect and made it easier to counteract rudder movements. After other controversial aircraft incident potentially involving the rudder on the 737 and even on a British Airways 747 Boeing redesigned the rudder servo system and there have been no such incidents involving rudder thereafter.


Thanks to the Captain and crew members of Eastwind Airlines Flight 517, one of the longest aircraft investigations was solved and fruitful. These aircraft incidents prove that aviation industry is full of uncertainties. Even minor errors can be costly. However, it should be noted that these incidents happened more than two decades ago and aviation has undergone paramount improvement and development within the time frame.

To learn more in detail, I would recommend you to watch this youtube video. Click here.

Also find similar aircraft incident reviews here http://geniuserc.com/topics/engineering-and-technology/aviation/aircraft-incident-review/

By Nishchal Poudel

Nishchal is currently studying Bachelor in Aerospace Engineering at IOE Pulchowk Campus, Nepal.

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