Archive for February, 2014

Mast moment

Monday, February 17th, 2014

A rigid (or sometimes called hinge-less) rotor system is capable of transmitting high bending forces to the main rotor shaft. When a pilot makes a cyclic movement causing the main rotor disc to tilt, the fuselage wants to follow. In flight, with a rigid rotor the mast bending moment is low. However, when the fuselage is in contact with the ground and cannot follow the main rotor disc the bending moment can be very high. 

This type of rotor system is used on the helicopters designed and built by the German manufacturer MBB (now Airbus Helicopters). Because large cyclic displacements on the ground have the potential to damage the mast assembly, a mast moment indicator (MMI) is installed. The gauge is a single dimension indicator that shows the total moment being applied to the mast. When the gauge reads high, the pilot has to figure out what direction to move the cyclic to reduce the mast moment. Over time, experience makes knowing how to keep the mast moment low a natural reaction, however, pilots new to these types of helicopters would have to be very careful not to exceed the limit. Recently, to help reduce any possible confusion a new style gauge has been developed. It is two dimensional (using a circle instead of a straight line) which makes knowing the correct direction to move the cyclic control easier. 

Normal pick-ups and set-downs require care as to not exceed the limits on the MMI. Generally, this is not difficult. However, slope landings and running landings can be more challenging. In these situations, the pilot needs to be comfortable with the MMI being close to limits and making very small cyclic adjustments. If a limit is exceeded, the amount (in percentage) and duration dictate how extensive an inspection or repair will be.

Older style MMI

Older style MMI

                   

Newer electronic single dimension MMI

Newer electronic single dimension MMI

 

 

 

 

Latest two dimension MMI

Latest two dimension MMI

Teaching autorotations

Saturday, February 1st, 2014

One of the most critical maneuvers that helicopter CFIs perform with their students is autorotations. It requires precision, timing and the ability to multitask.  Rotor RPM, airspeed and trim must all be maintained within allowed parameters while simultaneously finding a suitable landing area and maneuvering the helicopter into the wind. From 500 feet above ground level, a student has 20 to 30 seconds to process and manage all the factors and make the right decisions to achieve a successful outcome.

Allowing a student to perform an autorotation requires constant vigilance from the instructor. The best way for students to learn is by doing as much of the maneuver as possible, however, the instructor does not always have a lot of time to decide to take the controls before the student gets the helicopter in an unrecoverable situation. Sometimes, the difference between a successful practice autorotation and an accident is just a second or two.

During the first 2 months of 2012 three accidents happened from practice autorotations and the NTSB issued the following probable causes:

  • The flight instructor’s delayed remedial action during the pilot-receiving-instruction’s practice autorotation that developed a high rate of descent. Contributing to the accident was the pilot-receiving-instruction’s improper control inputs during the practice autorotation. 
  • The flight instructor’s failure to apply power during a practice autorotation in order to arrest a high rate of descent, which resulted in an in-flight collision with terrain. 

These two happened in a Robinson R22 and a R44. However, the following is from an AS350 with a more experienced instructor. 

  • The flight instructor’s improper use of the collective control during a practice hovering autorotation, which resulted in a hard landing.

Even an excellent and experienced instructor who gets distracted, even for just a second or less, can damage an aircraft. Full touchdown autorotations (that is, not bringing the engine back in before ground contact) add another level of risk. Fortunately, most accidents that happen from practice autorotations are not fatal.

 

NTSB accident references:

NTSB Identification: WPR12TA120

NTSB Identification: ERA12CA179

NTSB Identification: ERA12CA137