Trying to decipher AS350B2 power check chart

[Shawn Coyle]

Sorry to bother folks with what should be a simple question -
Trying to understand the performance charts on the AS350B2, particularly the engine power check chart.
The chart has an example calculation, but the manual doesn't describe the process. Everything is pretty clear except for the top of the chart where there's a curious set of lines that reverse and end up with a different Ng than what I have to assume is the original value recorded. Eurocopter's normally good training material doesn't appear to cover this.
What I'm trying to find out is why the reversing of the lines?https://docs.google.com/file/d/0B2jY9QR4vO_pQVcwUWJsc2JjTWs/edit?pli=1

[simps7b2]

The image in this post can be viewed here: https://docs.google.com/file/d/0B2jY9QR4vO_pQVcwUWJsc2JjTWs/edit?pli=1. I think everyone has access to it.

[Arm out the window]

Is it as simple as this?

Enter Nr & Tq, where they join, go up, follow the lines until you hit PA, then extend a line from there up as far as you like (line 1);

Then come in from the other direction with OAT across until it hits Ng, go straight down to the 'correct/incorrect' guiding line and then across to the left until you hit line 1;

if the intersection falls in the 'correct' part it's OK?

Don't know, but that seems like it could be what they're suggesting with the reversed lines.

[TeeS]

Hi Shawn

That is weird, presumably there is no way of knowing the position of the top right point unless there is another chart or graph that gives a 'target Ng' for OAT? Would the drop down to the incorrect/correct line then give you an automatic correction for DA? Thinking aloud here rather than offering a solution.

TeeS

[Devil 49]

I am with Mr Coyle in being confused by this format although I've used it (successfully, I guess) for 30 years. What happens when the line from observed parameters (bottom up) aligns with the line from critical parameters (top down)? I have always rechecked to break the tie.
It is really hard to be precise plotting this as one must parallel curves and the marks made have width. Digital, a spreadsheet for instance, would be concise.

My best guess is that Gallic logic required that plotting observed line and decision line points for intersection be 2 different operations.

[jymil]

As already mentioned before, the top right point is the intersection of OAT and Ng. From this point you go either down or up to the line dividing the correct/incorrect sections. Then, you go either left or right to intersect the line coming up from the Hp chart. Point P does not signify any particular Ng value, it only signifies either "correct" or "incorrect".

Maybe it's also helpful to construct an "incorrect" example. Take the values from the original example and use Hp=3.000ft instead. The OAT/Ng intersection would be the same, so you go right and down to the correct/incorrect line the same way. But to intersect now with the line coming up from the Hp diagram, you have to go right into the "incorrect" section, meaning you pulled too much power for that altitude.

[JimEli]

I can’t explain why the manufacturer constructed this chart with the correct/incorrect shaded areas and the bizarre and convoluted path needed to determine a valid Power Assurance Check. The entire process could have been established with a much easier and less confusing process.

Regardless, if the vertical line derived from the bottom is to the LEFT of the vertical line derived from the OAT/Ng intersection, the engine is “CORRECT”.

Shawn, I have more info on this if you're interested.

[Jack Carson]

The AS-350B-2 is powered by an Arriel 1D1. I believe that this is and Ng limited power plant. As such the Ng limit is calculated as Ng divided by the square root of Ø, where θ represents the ratio of the air stream temperature at a chosen reference station relative to sea level standard atmospheric conditions.

The jog in the chart allows for the increased Ng capabilities of the engine at higher ambient temperatures. This explanation comes with the disclaimer that I had to dig far deeper into the bowls of experience and past training than would be considered reasonable.

[RVDT]

Arm out the Widow has the correct method.

1.2 Use of the chart
Read the chart in the direction indicated by the arrows, entering as
follows: -
-Torque -NR -Altitude
and
-O.A.T. -Ng
Follow the example to locate point "P".
Engine power is O.K. if point "P" is located in the region marked
"CORRECT"

Point "P" has no relation to the values on the upper chart other than the "CORRECT/INCORRECT" line. On other engines the limit would be TEMP but not on an Arriel as Mr Carson sez.

Some insight here (http://home.earthlink.net/~jameseli/Misc/Heli_Perf.pdf).

[Shawn Coyle]

Thnaks to all who responded. With some help from Roberto Bouchard of Eurosafety in Dallas, we now understand how to use it. But it could be better explained in the manual...