Archive for May, 2013

Fit for flight

Sunday, May 19th, 2013

Every so often I come across an accident that really makes me stop and think. Many of these can be a learning experience and some are just hard to understand. 

According to the NTSB, on July 22, 2010 a Eurocopter AS 350 B2 helicopter impacted trees near Kingfisher, Oklahoma. The commercial pilot and one flight nurse were fatally injured and one paramedic flight nurse was seriously injured. 

A Global Positioning System (GPS) device recovered from the accident scene revealed the helicopter was cruising at approximately 130 knots and about 200 to 300 feet above ground level. Seconds before impact, the helicopter descended at 385 feet per minute, followed by a descent rate of 1,890 feet per minute two seconds later. The location and altitude of the helicopter, as recorded by the GPS corresponded to the location rotor impact marks with the trees. 

In an interview with the surviving paramedic flight nurse, he recalled that during the flight, the left side door had come unlatched and was slightly ajar. The paramedic informed the pilot that he was getting out of his seat to close the door and secure the handle. The pilot acknowledged the paramedic. After securing the handle, the paramedic stated that he had sat back down and begun to gather his seatbelt when a conversation began about another pilot flying on a coyote hunt. The paramedic reported that the pilot made a statement similar to “like this… (with some laughter)” and made a nose down control input. He reported that the pilot pulled up on the collective and the helicopter struck a tree. During the ground impact, the paramedic, who was not secured in his seat, was thrown through the windscreen; the paramedic crawled away from the wreckage and dialed 911 on his cell phone. 

The pilot, age 56, held a commercial pilot certificate for airplane single-engine land, instrument airplane, rotorcraft-helicopter, and instrument helicopter. He held a second class medical certificate issued February 8, 2010. On the pilot’s last application for a medical certificate he reported having accumulated 12,241 hours, with 119 hours logged with the preceding six months. Of note, the pilot reported that he was not currently using any medications. 

An autopsy was performed on the pilot and toxicology noted the following: 

  • 39.31 (ug/ml, ug/g) Acetaminophen detected in Urine
  • Chlorpheniramine detected in Blood
  • Chlorpheniramine detected in Urine
  • 0.198 (ug/ml, ug/g) Diazepam detected in Blood
  • 0.026 (ug/mL, ug/g) Dihydrocodeine detected in Blood
  • 1.026 (ug/mL, ug/g) Dihydrocodeine detected in Urine
  • 0.15 (ug/ml, ug/g) Hydrocodone detected in Blood
  • 4.112 (ug/ml, ug/g) Hydrocodone detected in Urine
  • 0.302 (ug/mL, ug/g) Hydromorphone detected in Urine
  • 0.322 (ug/ml, ug/g) Nordiazepam detected in Blood
  • 0.629 (ug/ml, ug/g) Nordiazepam detected in Urine
  • 0.011 (ug/ml, ug/g) Oxazepam detected in Blood
  • 2.169 (ug/ml, ug/g) Oxazepam detected in Urine
  • 1.569 (ug/ml, ug/g) Temazepam detected in Urine 

A review of the pilot’s medical history found that the pilot was being treated for several medical conditions and had been prescribed multiple medications since at least 2007. In April 23, 2007, the pilot reported to his personal physician that he had bronchitis, hypertension, and sleep apnea, and after his visit, he was prescribed the following medications: Nexium (for gastroesophageal reflux), Caduet (for hypertension), Flexeril (sedating muscle relaxant), Lortab (hydrocodone and acetaminophen; narcotic pain medication), Lunesta (for sleep disturbance), and Requip (for restless leg syndrome). The pilot continued to report to his personal physician that he experienced increased pain and was prescribed stronger pain medications, to include prescription narcotics and benzodiazepines. In addition, steroid joint injections were applied to his right knee and shoulder to treat persistent pain. The last documented visit, February 25, 2010, the pilot was prescribed the following: Caduet (for hypertension), omeprazole (for gastroesophageal reflux); Meloxicam (a non-steroidal anti-inflammatory); Lunesta (sleep aid); Norco (10/325 hydrocodone/acetaminophen combination two tablets three times a day); baclofen (a muscle relaxant, 10 mg three times a day) and Valium (diazepam, a benzodiazepine, 10 mg three times a day). In addition to his prescribed medications, chlorpheniramine, an over-the-counter sedating antihistamine medication was also detected in the toxicology. There was no evidence that the pilot’s sleep apnea had been treated prior to the accident. In addition, the pilot did not report any of his conditions and prescription medications to the FAA, to the certificate holder, or to the operator.

 

Delta-3

Monday, May 6th, 2013

Dissymmetry of lift occurs when a rotor system is flown edge-wise through the air. With helicopters, many times these discussions center on the main rotor system. However, this aerodynamic condition also affects the tail rotor.

Just like the main rotor, a tail rotor will equalize lift by flapping. However, most tail rotor flapping takes advantage of the Delta-3 effect. Also known as pitch-flap coupling or K-Link (French term). This effect is achieved by having the pitch horn on a different plane than the flapping hinge, which mechanically changes the pitch angle of the blade as it flaps. The amount of the delta-3 offset is measured in degrees and determined by design engineers after considering many factors. This offset can be also be accomplished by using a Delta-3 hinge (setting the hinge at an angle to the chord of the blade). In either case, when the advancing blade (the blade that experiences a higher relative wind) starts to flap the offset lengthens the distance between the blade’s pitch horn and the pitch link’s attach point. This forces the pitch link to pull the blade’s pitch horn closer, thereby reducing its pitch angle. On the retreating side, the distance is shortened and the pitch link forces the pitch horn further away, increasing the blade’s pitch angle. This effect minimizes flapping in order to control dissymmetry of lift on the tail rotor.

This can be demonstrated by moving a tail rotor blade with a delta-3 hinge through its flapping range and observing the pitch angle changes as you manually flap the blade.

 

AS350 tail rotor

AS350 tail rotor