Sunday, December 1, 2013
Tipping the Scales
After many hours in the cockpit, it’s not uncommon for a good helicopter pilot to become very in-tune with their machine and its abilities. Getting to know your aircraft to this level and being able to operate it safely at or near its limits is commendable evidence of countless hours of flying. But don’t ever use it as a substitute for computing the required performance, weight, and balance checks prior to your flight. A quick check of the NTSB accident reports will show that failure to do so continues to be a factor in many fatal accidents, and therefore worthy of yet another review.
A helicopter’s maximum allowable gross weight is determined by many design considerations; but this weight is not necessarily always the maximum allowable operational weight. The maximum operational weight will depend on the ambient conditions (i.e., density altitude) at the time of the flight. The conditions at your takeoff location may not match the conditions at your landing location either, so be wary. The performance charts in the flight manual combine airframe (rotor) performance with engine performance and usually present this to you in a hover ceiling chart.
Be aware that this assumes your engine is healthy and is producing its rated power, and only shows you what maximum weight the helicopter can be hovered at. It does not show you if you have any performance margin remaining to deal with winds, maneuvering in the hover, transitioning to forward flight, or to stop a rate of descent in a vertical landing.
Assuming that there is enough of a fudge factor built into the performance charts for you to ballpark your maximum operational weight is like lining up the holes in the “Swiss cheese” that can take you straight to an accident scene. At high-density altitudes, operating very close to your power limitations can tip the scales toward bad things fast – too fast to be recoverable. A drooping rotor coupled with an approaching tree line will have a predictable outcome. Alternatively, you cannot assume that you are within your max gross weight limit just by your ability to achieve takeoff power. In fact, you may be over gross and still not know it. You risk over-stressing components and possible structural failure in flight due to excessive load factor in maneuvering, strong wind gusts, or turbulence.
Ensuring you are within your operational weight limitation is one half of the equation, so to speak. The other half is ensuring the entire flight is conducted within the aircraft’s allowable center of gravity (CG) range. The CG can change throughout the flight as payload changes or as fuel is burned, so always compute zero-fuel CG! All forces affecting roll, pitch, and yaw create moments which act about the CG. The amount of control power available in each direction will depend on CG location. Control power is the amount of aircraft response achievable with the available control margin.
Running out of stick or pedal is like a child on the high end of a see-saw running out of seat behind them to scoot back on, when trying to bring the see-saw back down. An excessive forward CG will require more aft cyclic just to remain in trim, reducing the control margin, or the amount of aft cyclic remaining. This can make it difficult to slow down, or flare properly in an autorotation.
Similarly, too far aft of a CG will reduce your forward cyclic margin. Gusty conditions that cause the rotor to flap back may then be impossible to correct, causing loss of control. Also, as the CG moves aft, the tail rotor must work harder to maintain directional control, decreasing pedal margins especially in a hover or autorotation.
Inattention to lateral CG limits can decrease control available in hoisting, crosswinds or slope landings. Vertical CG changes, though not usually calculated, can affect handling qualities, showing up as “dutch roll,” or coupled yawing/rolling tendencies.
Flying thousands of hours during all sorts of missions does not equate to having superior flight discipline. Trying to manage risk with experience alone is a poor approach to conducting a safe flight. The trouble with this approach is that things might be “all good” – until they’re not.
With smartphones and tablets becoming prevalent in the cockpit, it’s easier than ever to conduct proper preflight performance calculations. Parts 27 and 29 of the Federal Aviation Regulations (FARs) do not give an exact amount of control margin required for certification of helicopters at critical weights or CGs, but the generally accepted amount is 10 percent. Without my orange flight suit and a fully-instrumented experimental aircraft strapped to me, I don’t want to be the guy finding out exactly where my engines top out, or guessing if I’ve got 10 percent control margin left on any given flight. Knowledge is power, but the discipline to say “No” is king.