Commercial Pilot Flight Lesson: It's much cooler up here at the LZ than it is down in Vegas, but the pressure is of course lower. Lars explains how to predict high density altitude takeoff performance using atmospheric data off the instrument panel in conjunction with the power tables. Set 29.92" in the Kollsman window and read the pressure altitude directly. In this case, we should just barely be able to achieve Max Takeoff Power. However we shouldn't even need max continuous power to hover in ground effect (which we can easily pull) because we are pretty light due to our preflight planning. We're only about 45 minutes from Vegas.
The power available vs. power required ratio is almost always a factor in high density altitude helicopter accidents - and unfortunately there are too many examples of these. The accidents highlight the necessity to figure out in advance what the helicopter can and cannot do. It's easy to know you'll be able to safely depart in a training scenario like this one because we just landed a little while ago and hovered with no problem. However, it's important for the student to understand the need not to fall into the trap of complacency just because it's a training flight. You still need to go through the process of determining takeoff performance because if you don't know what you are doing now, then what's going to happen after training is over? Imagine an operational scenario in which you are picking up a load on that pinnacle, or where you arrived in the morning when it was cold but are departing in the afternoon when it's hot. The steps learned here are how you'll safely handle those situations.
The performance charts provided by the Robinson RFM are of course sufficient to get the job done, but it's always good to have a full understanding of what these tables are actually telling you about what kind of performance you can expect. Toward that end, it would be nice to know what the approximate station pressure is. It's easy if the engine isn't running. You just read it off the manifold pressure gauge. Otherwise, if you happen to know the elevation of your LZ (e.g. by looking at a topographical map or preflight use of Google Earth), simply adjust the altimeter until it reads the station elevation. The sea level pressure can then be read in the Kollsmann window. To get station pressure, subtract the ole' "1 inch per thousand." If you don't know how high you are beforehand, you can use real-time GPS data from almost any available source to get the elevation - from a contour chart or survey map, from your avionics if you have them, from a GPS enabled wristwatch, or from a Bad Elf or Stratus unit. In this case we neglected to check the manifold pressure before starting up but, as Lars mentions, that's actually good because now we get to practice figuring it out another way. Understand that you aren't actually "required" to do this calculation, but in serious high-altitude situations it's very good information to have because it helps predict approximately how much power you'll be able to pull (slightly less than the existing static pressure once the engine is running at full throttle. With some experience you'll get to know the amount of loss pretty accurately.) Comparing this estimate to RFM data then gives you an idea of how much margin you'll have from full throttle - NOT ACCORDING TO THE TABLES, but according to the helicopter itself in the actual situation. Yes, the data should be about the same, but if there's a significant difference then you have the opportunity to investigate the discrepancy before you have an accident. Don't underestimate the potential impact of the correct altimeter setting on your performance. Without it, you could be hundreds of feet off, even 1,000' is not out of the question. That's plenty enough to cause a problem in a marginal performance situation.
The mindset that a helicopter pilot needs to have is: "It's not going to happen to me because I know that it can." This means you take the all necessary steps, and then maybe even a couple that aren't "necessary" on top of that to make sure that you don't screw it up. The life you save may be your own.
