Archive for April, 2013

Eurocopter’s AStar

Thursday, April 25th, 2013

In the late 1970s, Aerospatiale introduced the AS350B as a replacement for the company’s Alouette II helicopter. Named Ecureuil (Squirrel) it used a Turbomeca Arriel 1B engine rated at 641 shp. For the U.S. market Aerospatiale gave it the name AStar and the model number AS350C. Both models had a maximum gross weight of 4,300 pounds, however, the C model used the Lycoming LTS 101-600A engine rated at 592 shaft horsepower (shp) for takeoff. Six months later the helicopter was upgraded to the D model with the installation of the Lycoming LTS 101-600A-2 engine, boosting its takeoff rating to 615 shp.

In 1987, Aerospatiale discontinued the D model and upgraded the B to a B1. This new version had a more powerful Arriel 1D (684 shp) engine and a higher gross weight of 4,850 pounds. The rotor system was upgraded with larger (more inertia) asymmetrical rotor blades and rotor rpm was increased slightly. This resulted in increased performance and better autorotation characteristics. The company also produced the BA model that had the larger blades, but retained the B model’s Arriel 1B engine. Three years later the B1 was replaced with the B2, using a more powerful Arriel 1D1 (712 shp) engine and gross weight jumped to 4,961 pounds. The B2’s cruise speed at MCP (maximum continuous power) is 133 knots. In 1992, Aerospatiale merged with MBB to form Eurocopter.

In the late 1990s Eurocopter introduced the B3, a high altitude version.  It was powered by an Arriel 2B engine equipped with a single channel DECU (Digital Engine Control Unit) with a mechanical backup system. Although the gross weight remained the same, take off power increased to 747 shp. This was followed by a variant using the 2B1 engine with a dual channel FADEC (Full Authority Digital Engine Control). This version had a dual hydraulic system available as an option which when combined with high skid gear allows a gross weight increase to 5,225 pounds. The latest version, B3e, was introduced in late 2011 and is outfitted with a dual channel FADEC equipped Arriel 2D engine. Take off power rating jumped to 847 shp boosting its MCP cruise speed up to 137 knots and adding extra lifting capability.

CDPH0591-086

Vibration analysis

Friday, April 12th, 2013

All helicopters have an inherent vibration. The type and intensity varies as a function of rotor design and isolation systems. Understanding basic vibration levels and being alert to changes can be an important tool for preventing fatal accidents. Difficulty with tracking and balancing the main rotor system is a condition that should raise concern with pilots and mechanics.

Two accidents involving Robinson R22 helicopters, one in Australia in June, 2003 and the other in Israel in February, 2004, involved increasing vibration levels in the main rotor system. In both aircraft, the vibrations were corrected with track and balance only to reappear a short time later. In fact, the accident in Israel happened during one of the track and balance flights. In both cases, investigations revealed that corrosion from water penetration initiated a fatigue crack in the main rotor blades.

More than a year prior to the first accident, Robinson Helicopter released a Service Letter (SL-53) regarding potential development of main rotor blade fatigue cracks when the helicopter is operated under conditions where the loads on the main rotor exceed the design limits. In part the letter stated, “The first indication of a fatigue crack in progress may be a rotor that will not stay balanced after being adjusted.”

Then in July of 2003 Robinson Helicopter issued a R22 Safety Notice again stating that vibrations that reappear after tracking and balancing the main rotor system should be consider suspect.

Safety Notice SN-39

UNUSUAL VIBRATION CAN INDICATE A MAIN ROTOR BLADE CRACK

A catastrophic rotor blade fatigue failure can be averted if pilots and mechanics are alert to early indications of a fatigue crack. Although a crack may be internal to blade structure and not visible, it will likely cause a significant increase in rotor vibration several flight hours prior to final failure. If a rotor is smooth after balancing but then goes out of balance again within a few flights it should be considered suspect.

Knowing this information is important to help pilots and mechanics prevent future accidents.

In-flight vibrations

Wednesday, April 3rd, 2013

When a critical component in a helicopter’s main rotor system fails in flight, the resulting accident is almost always fatal. How much warning, if any, does a pilot get with these kinds of failures? Unfortunately, the vast majority of helicopters do not have cockpit voice recorders and unless the pilot can provide ATC with details, it can be hard to understand exactly what happened. Even if the pilot is in contact with an air traffic controller, an emergency situation leaves little time to completely explain a problem. Consequently, the crash of a Bell 212 equipped with a cockpit voice recorder near Philadelphia, Mississippi is unique in that it provides some insight as to what the flight crew knew. The helicopter was destroyed and the airline transport-rated pilot and one passenger (who was employed by the owner as a mechanic) were fatally injured.

The transcript of communications recorded on the cockpit voice recorder showed that about 18 minutes before the accident, the passenger 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 1 minute, 30 seconds before the accident, the pilot asked the passenger, “Has this vertical 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 seconds 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.

The lesson in this accident is that any unexplained vibration should be investigated on the ground until the source is found and corrected. Some parts and bearings that become loose can experience exponential wearing and fretting and quickly reach a failure point.