Jun 19 2017

Not all Twins are Alike Posted 7 years 187 days ago ago by Randy Mains

“But all twins are not alike”, I said to the air medical flight doctor who is very keen to make it mandatory that all air medical programs in America operate twin-engine helicopters. He replied, “I wasn’t aware of that.”

                So, what are the differences?  It all has to do with the weight-to-horsepower ratio of the machine and the ability to either land safely on one engine or fly away.  Helicopters are categorized by the FAA as Performance Class 1, 2 or 3.

                Performance Class 1 is defined as those helicopters with performance such that, in the event of failure of the critical power unit, the helicopter is able to land within the rejected take-off distance available or safely continue the flight to an appropriate landing area, depending on when the failure occurs.

                To be operated in Performance Class 1, a helicopter must be certified in Category A, which is a design requirement meaning it must be equipped with at least two engines, and also have a certain number of safety-related equipment items, as well as redundant backup for control, lubrication, etc. Category A helicopters must offer the performance needed to guarantee that, in case of an engine failure, the flight can continue safely. 
               

 Under Performance Class 1 conditions, the helicopter can manage the failure of one of its two engines at any given moment while maintaining satisfactory safety criteria, especially during the takeoff or landing phases.

                Performance Class 2 is a helicopter also certified Category A, but a certain "exposure time" to the engine failure is allowed. When the engine failure occurs early during takeoff, or late during landing, a forced landing may be necessary depending on power available from the remaining engine. What that means is Performance Class 2 offers a lesser safety guarantee than Performance Class 1. However, if the failure should occur in flight, the helicopter’s performance means that it can still continue the flight as long as it is not too high or too heavy to overwhelm the remaining engine. 

                Performance Class 3 helicopters are such that in the event of a power unit failure at any time during the flight, a forced landing may be required in a multi-engine helicopter (Think heavy helicopter where the ‘good engine’ does not have the horsepower to keep it aloft.) but will definitely be required in a single-engine helicopter.

                Category A means multi-engine helicopters with engine and system isolation and Flight Manual performance based on a critical engine failure concept providing adequate surface area and performance capability for continued safe flight if an engine fails. In other words, in addition to making sure you have power available (by restricting maximum all up takeoff weight, it provides space for rejected takeoffs and landings, and obstacle clearance.

                Category B means single- or multi-engine helicopters not fully meeting Category A criteria. They are not guaranteed to stay airborne if an engine fails and an unscheduled landing is assumed, possibly with some damage.  Here’s a good personal example:  

                On the night of January 26, 1982 I was flying an Alouette III in the mountains east of Miramar Marine Air Station in San Diego on a HEMS mission trying to find my way up valleys in low cloud.  I decided that the weather was too bad to continue and called our hospital dispatch to say we were turning around.

                I dialed up the Miramar frequency to inform the tower controller we were going to re-enter their airspace.  He came back with an urgent request saying he had an H-61 (Sikorsky) helicopter in trouble saying that one of its engine’s had exploded and couldn’t maintain altitude on the good engine, that he was in a slow descent into the mountains and that the pilot didn’t know his exact position.  He told me, “He’s too low for us to pick him up on radar.  Can you see if you can find him?”

                I began a turn back into the mountains and bad weather, and transmitted,  “Roger Miramar, I’ll see what I can do.  Any idea where he might be?”
                The controller gave me a radial and distance from the Julian VOR.  I did a quick mental calculation and turned on a heading back into the mountains to see if I could spot him.

                It was extremely dark and the weather wasn’t any better than when I turned around last time.  These were the days before NVGs so I could only just make out the terrain below and around me.  I flew for about five minutes when the Miramar controller radioed,   “He says he sees your red anti-collision light Life Flight One.  Can you guide him out?”

                Because he was flying a military aircraft we didn’t have a common radio frequency to talk to one another so we had to relay through Miramar tower:  “Tell him to follow me. I’ll show him the way home.”
                “Roger.”

                I led the much larger H-61 helicopter out of the mountains, his good engine delivering only enough power to keep him from flying into the ground, mainly because the terrain was falling away as we flew west so he managed to avoid flying into the ground. Over my radio, I heard,  “He sends you his thanks, Life Flight One and says he doesn’t know how he would have made it out without your help.”

                “My pleasure, Miramar. We’re flying back to the hospital now.”  

                That story is a good example of a twin-engine helicopter, in this case. unable to meet either Performance Class 1 or Performance Class 2 during the enroute phase of flight.

                My personal view is that no matter what their capability Performance Class 1, 2 or 3, I much prefer the peace of mind flying a helicopter with two engines rather than just one because, as in the Marine crew’s case: You never know when you’ll need that second engine.

Randy Mains is an author, public speaker, and a CRM/AMRM consultant who works in the helicopter industry after a long career of aviation adventure. He currently serves as chief CRM/AMRM instructor for Oregon Aero. He may be contacted at [email protected]