Archive for September, 2011

Translating tendency

Friday, September 23rd, 2011

If you have ever closely watched a hovering helicopter, you might have noticed that most times the skids are not level with the ground. In other words, one of the helicopter’s skids is lower than the other. Although wind and loading can cause this, the tail rotor thrust determines the base line for either right skid low or left skid low. Tail rotor thrust tends to make the helicopter drift in the same direction and is called translating tendency.

The tail rotor is designed to produce thrust to oppose the torque that tries to spin the helicopter in the opposite direction of the main rotor. Some of this thrust applies a force to the fuselage that causes the helicopter to drift laterally in the same direction. A tilt in the main rotor causes a small sideward thrust opposite the tail rotor to counteract the drift. The tilt can be accomplished by mounting the transmission at a slight angle or designing the flight control system to tilt the rotor disc when the cyclic control is centered.

The direction the rotor spins makes a difference. In a system the turns clockwise when viewed from above, the tail rotor thrust causes the helicopter to drift to the left. Tilting the main rotor disc to the right to counter this causes the right skid to hang low. A counterclockwise turning system will cause a right drift and a left tilt making the helicopter hover left skid low.

Fuel exhaustion

Thursday, September 15th, 2011

I have always felt that pilots can learn a lot by reading and reviewing past accidents. One type of accident that seems to happen over and over is running a helicopter out of fuel. I just don’t understand this, especially in an aircraft that can just about land anywhere. The following accident happened almost nine years ago.

At night on October 15, 2002 a Schweizer 269C experienced an engine failure due to fuel exhaustion and was substantially damaged during the ensuing hard landing. The CFI pilot was providing night VFR cross-country instruction to a student. They had discussed the low fuel situation, but elected not to refuel because neither had a credit card. On the last leg of their flight the low fuel light illuminated followed a few minutes later by complete loss of engine power. During the autorotation, the helicopter struck trees and the tail boom separated from the airframe. Miraculously, neither pilot was injured.

Then, just a few weeks ago a Eurocopter AS-350-B2 helicopter sustained substantial damage when it impacted terrain during an autorotation following a loss of power. The pilot, flight nurse, flight paramedic and patient received fatal injuries.

The purpose of the flight was an inter-hospital transport.  At 1730, the pilot reported to the communication center that he departed the helicopter’s base with two hours of fuel and 3 persons onboard and was en route to the hospital. Approximately 28 minutes later the helicopter landed at the hospital helipad to pick up the patient. While the helicopter was shut down on the helipad, the pilot contacted the company’s communication center by telephone and notified them that about half way through the flight he realized that he did not have as much fuel onboard as he originally thought. After a discussion about possible fueling and re-routing options, the pilot elected to stop en route for fuel and then proceed to the hospital helipad to drop off the patient.

Shortly after departing from the hospital helipad the pilot contacted the company’s communication center and reported that he had 45 minutes of fuel and 4 persons onboard. Thirty minutes later the helicopter crashed in a farm field about 1.7 nm miles north-northeast of the intended fuel stop. There was no post impact fire.

These kinds of accidents are very frustrating because they are so preventable.

Airborne infrared cameras

Tuesday, September 6th, 2011

Airborne law enforcement helicopters typically fly with a pilot and a tactical flight officer (TFO). The TFO communicates with ground units and operates the surveillance equipment. The pilot communicates with ATC and flies specific orbits and altitudes that maximize the TFO’s visibility of the suspect or scene. Both of these positions require good communications skills and the ability to work together as a team.

Recently I flew with Texas DPS in their Dallas based AS350 helicopter. The TFO was Clay Lacey and the pilot was Jim Rohrman. One of the interesting pieces of equipment they used was a gyro stabilized FLIR camera. The term FLIR stands for forward looking infrared, and is used to refer to a thermal imaging camera. FLIR cameras allow the TFO to see in total darkness by producing viewable images of invisible infrared energy. Infrared energy is part of the electromagnetic spectrum, which we perceive as heat, so it is invisible to the naked eye. Some level of thermal energy is emitted from all people, objects, and material.

On the night I flew with Texas DPS we circled a fight in an apartment complex parking lot. Hearing the helicopter overhead some of the subjects walked into dark areas between the buildings probably thinking they couldn’t be seen. They were perfectly clear on the FLIR screen. Although no one ran, Clay explained that when they do it is very easy to follow them at night and direct ground units to apprehend them. In one case as ground units were surrounding a suspect he reached into his pants, removed a gun and threw it onto a building roof. The gun retained some of the heat from his body and the TFO caught it on his FLIR camera and directed the ground units to find it.

Both Clay and Jim have won awards for their work in apprehending criminals using a FLIR camera. To see a video of their work and how effective these cameras and crews are at apprehending suspects follow these links: