Archive for November, 2011

Bell’s corporate helicopter

Wednesday, November 30th, 2011

In 1979 Bell Helicopter certified the Model 222 helicopter to target the corporate market. Although it had a sleek corporate look, the helicopter struggled to find acceptance in the business world. This was due to reliability problems with the Lycoming LTS 101 engines and the two-blade rotor design that could not achieve an acceptable level of smoothness.

As a result, Bell began working on a new helicopter that would use advanced technologies to improve the engines, rotor system, and cockpit. In 1994, with the new design not quite ready, Bell introduced the model 230 with the more powerful Allison 250-C30 engines and numerous small refinements as an interim fix for many of the 222’s problems.

Finally, in early 1996, Bell certified its next generation helicopter, the model 430, and stopped production of the 230. The Bell 430 has a bearingless four-blade composite rotor system combined with Liquid Inertia Vibration Eliminators (LIVE) mounts on the transmission that give it a smooth ride. The engines were upgraded to the more powerful FADEC controlled Allison 250-C40B and a glass cockpit was available. The airframe was stretched 18 inches making for a larger cabin and the gross weight went from 8400 lbs. to 9300 lbs. One of my favorite features is the pilot’s side (right seat) collective control that moves forward and aft in a horizontal arc instead of up and down. To me it felt more natural.

Although, a little underpowered Bell did a good job with this helicopter, it was smooth, stable and fun to fly and had gained acceptance with corporate and EMS operators. Unfortunately, in January 2008 after building 136 helicopters, Bell announced they are stopping production of the 430 citing that it is optimizing its commercial product line to better serve its customer base and accelerate deliveries of its high-demand aircraft.

Bell 430

Bell 222

Main rotor vibrations

Monday, November 21st, 2011

When a critical component in a helicopter’s main rotor system fails in flight, how much warning, if any, does a pilot get with these kinds of failures? Unfortunately, helicopters typically do not have cockpit voice recorders (CVR) so it can be hard to understand exactly what happened. Consequently, the following accident is unique in that it provides some insight as to what the flight crew knew. 

On Nov. 27, 1999, a CVR equipped Bell 212 crashed near Philadelphia, Miss. The transcript of communications recorded on the cockpit voice recorder showed that about 18 min. before the accident, the passenger (who was also the aircraft’s mechanic) stated to the pilot, “Boy, those catfish are going crazy down there, aren’t they?” 

“Yep,” the pilot responded, “must have been the vibrations from the helicopter.” 

About 2 min. later, the passenger and pilot discuss sighting deer in a field. About 1 min., 30 sec. before the accident, the pilot asked the passenger, “Has this vertical (a term used to describe a vibration that moves up and down) just gotten in here or has it been here for a while?” 

“We haven’t had any verticals at all,” the passenger replied. 

“We do now,” the pilot said. 

“Yeah, well it started right after we left back there,” the passenger said. “I think it maybe, ah, that’s why I was thinking it was the air.” 

About 20 sec. later, the passenger stated that another person had tracked the helicopter’s blades before they left and that he was commenting on how smooth it was. Forty seconds after that, the pilot said, “This stuff is getting worse.” 

The recording then ended. 

The National Transportation Safety Board determined the probable cause of this accident was the failure of the pilot and company maintenance personnel during preflight and periodic inspections to identify the signs of fretting and looseness in the red main rotor blade pitch change horn to main rotor blade grip attachment. As a result, the NTSB found, the helicopter was allowed to continue in service with a loose pitch change horn, which led to separation of the pitch change horn from the blade grip and the in-flight breakup of the helicopter after the main rotor struck the tail boom. Contributing to the accident, the safety board said was the pilot’s failure to respond to increased vibration in the main rotor system and land immediately. 

Interesting to note is that the pilot and mechanic were aware of the vibration, but apparently never considered a precautionary landing. Any pilot would land immediately when a sudden and severe vibration occurs. But any unexplained vibration should warrant a precautionary landing. Some parts and bearings that become loose can experience exponential wearing and fretting and quickly reach a failure point. 

Many components on a helicopter can fail and still allow the pilot to make a safe landing. The main rotor system is normally not one of them. Thus, any abnormal low-frequency vibration felt in the airframe or through the flight controls should be treated with extreme caution.

Early skid gear

Thursday, November 10th, 2011

One of the early pioneers of helicopter flying was Carl Brady. In early 1947 he was crop dusting in a Stearman airplane when he saw a Bell 47B-3 spraying a field. Intrigued, he approached the owner and worked a deal out to get his helicopter pilot license. That same year he and two partners leased a couple of Bell 47B-3 helicopters and started their own operation.

Early helicopters, including the Bell 47B-3, used wheels for landing gear – probably a design borrowed from airplanes. Brady discovered that this was a bad idea for helicopters. He was known to tell a story that many consider the birth of skid type landing gear. It was 1948 and he and a former Bell mechanic were flying for the first time in Alaska. They discovered that the wheels would caster on rocky mountain tops or slopes causing the helicopter to roll downhill. To solve this problem they had a local sawmill cut two two-by-fours out of hardwood and using clothes line tied them to each wheel. It kept the wheels from castering and made landing on soft terrain much easier. Because there was no STC, they would fly their missions during the day and then remove the two-by-fours and fly back to town.

I have never read anything regarding the former Bell mechanic’s comments on the Alaskan adventure; however, two years later Bell introduced the Model 47D-1 with metal tube skid gear instead of wheels. This design became the standard for light helicopters for decades.

Mast rocking

Tuesday, November 1st, 2011

On May 12, 2009, a Robinson R-44 helicopter was damaged during a hard landing about 57 miles northwest of Iliamna, Alaska. The purpose of the flight was game management patrol for the Alaska State Troopers, Fish and Wildlife Service. After take-off from a ridge, about 300 feet above the ground, the helicopter was flying about 90 knots when the pilot felt an unusual medium-frequency vibration in the controls. The pilot told the NTSB that the vibrations turned to oscillations in both yaw and pitch to the point he felt the helicopter was going to come apart. He decided to make an immediate precautionary landing. During the descent the vibration increased and the helicopter landed hard causing the main rotor blades struck the tail boom.

The NTSB discovered that operators of the Robinson R44 helicopter were aware of similar events and that the condition had been dubbed “chugging.” According to Robinson Helicopter, tests determined that a mast rocking oscillation may develop during operation of the helicopter at high gross weight and about 90 to 100 knots. The oscillation was more of a “bucking” motion due to the fore-and-aft movement of the rotor mast. Tests also showed the tendency to enter the oscillation was exacerbated by a forward CG (within the CG envelope) and a 30 degree banked turn to the left. The oscillation is not divergent (that is, the main rotor blades do not diverge from their normal plane of rotation) and can be reduced by adding power. The oscillation is due to the firmness, or lack of firmness, of the transmission mounts. At the time there were no service alerts/bulletins referencing the phenomena or the remedies to resolve it.

On August 22, 2011 the NTSB issued the following safety recommendations to the FAA:

  • Require Robinson Helicopter to resolve the root cause of the mast-rocking vibration in the main rotor assembly to ensure that all applicable R44 helicopters are free of excessive vibrations in all flight regimes, as required by 14 Code of Federal Regulations Section 27.251, “Vibration.” (A-11-82)
  • Require Robinson Helicopter to maintain a database of all reported incidents of mast rocking in the main rotor assembly of R44 helicopters. (A-11-83)
  • Require Robinson Helicopter to issue a service letter to all approved service centers describing the mast-rocking vibration that can occur in the main rotor assembly of R44 helicopters and instructing service centers to report all incidents of mast rocking to the manufacturer. (A-11-84)
  • Require Robinson Helicopter to amend the R44 helicopter flight manual to inform pilots of the potential for mast-rocking vibration in the main rotor assembly and how to safely exit the condition. (A-11-85)
  • Require that the Robinson Helicopter R44 pilot training program be revised to provide pilot instruction in the recognition and mitigation of in-flight mast-rocking vibrations in the main rotor assembly. (A-11-86)

Most helicopter pilots are aware of mast bumping in semi-rigid (two-bladed) rotor systems, but this issue is new and not as well-known and raising awareness is important to safe operations.