Originally Posted by
212man
I tend to agree. A turbine helicopter has three possible limits - Ng/N1, Tq and T5/T4/TOT/ITT Add another engine and borderline ambient conditions, and you could be hitting two different limits more or less together. E.g max Ng on one engine while max torque on the other (I’m mainly talking about take off or OGE hover).. This is when an FLI really comes into its own. How many limits are there on an SE piston aircraft? Observe MAP limit per placard and?
Have you ever flown a piston helicopter? On both the Robinson and the Cabri, there are limits associated with manifold pressure, outside air temperature, time and available throttle.
On the Robinson the pilot must figure out what the manifold pressure limit is based on the current OAT. This is done by the crude method of looking at a table and interpolating the numbers (or just choosing the more conservative). The pilot must also keep track of time manually if using power in the 5 minute limit range (as determined by the aforementioned tabular lookup). On older Robinsons the table can be seen if you can crane your neck back far enough to see it on the overhead, and it's printed on the Robinson checklist card as well (or the POH if you really want to crack a book while flying). On newer Robinsons they place it on this cool little rotating cuff on the cyclic arm. And there is no indication of how much throttle you are using until you run out, something that can come as a surprise given the action of the throttle governor, unless you are in one of the newer serial numbers that has the "full throttle" indicator light (or an older one with that option, which seems to be a bit rare), which unfortunately gives you little warning of where you stand until you are standing there.
Contrast this with the MLI (see, I can learn
) on the Cabri, which takes all that into account and shows you continuously computed power in percent, with the 5 minute limit area being clearly designated by a yellow arc, and a convenient countdown timer when in the yellow arc. Plus the MLI conveniently switches between horsepower limited regimes and throttle limited regimes, the latter occurring at lower density altitudes, of course, thereby clearly indicating how much reserve remains in any situation.
Thus, while the limits and methods of measuring power differ in turbine and piston helicopters, they both can present sufficient complexity such that the presence of automation can be a significant boon to safe and easy operation. Certainly both types can be flown with round dial technology and rely more heavily on pilot computation, skill and attention, but better solutions are readily available nowadays.