Here Vernon, this relates to your question about the thrust rating. Read the Civil Thrust Rating. I admit I was never clear on this issue when you initially brought up the question.

 

https://en.m.wikipedia.org/wiki/Jet_engine_performance

Cheers for looking into but its not actually anything specific but rather what ISA + 10c means!

 

So would that mean its meaning ISA DEV? + 10

Yes. Basically, you would determine what the ISA temperature would be at the altitude you plan to fly. There are many online web sites with ISA tables you can reference.

 

ISA for 34000 feet, for example, would be -53C

 

Then, using a current upper winds forecast, determine what the ACTUAL temperature is at the altitude and location you plan to fly. If the temp at 34,000 feet on a given day and time is -43C, it is 10 degrees warmer than ISA, so you would use performance tables for "ISA +10". If the actual temp at 34000 feet was -63C (rare, but possible in polar regions), you would use tables for "ISA - 10"

 

 

The ISA deviation for a particular altitude is simply the difference between the actual temperature and the ISA temperature FOR THAT ALTITUDE.

 

ISA deviation can vary considerably as you ascend from the surface to higher altitudes. For instance, I am looking at the NOAA upper forecast graph for my location in New York State in the US for this exact time today, 01 Nov 2015.

 

At 6000 feet, the temp is 3C, which is the same as ISA for 6000 feet, so the deviation at 6000 feet is 0.

 

At 18000 feet, it is -13C which is ISA +8

 

At 27000 feet it is -34C, which is ISA +4

 

At 37000 feet it is -47C, which is ISA +9

 

Things cool off above 40,000 feet. At 42000 get it is -59C, which is ISA -3, and at 45000 feet, it is -63C, which is ISA -6.

 

Taken as a whole, this indicates an area of warmer-than-average air aloft at mid levels, which is due to the presence of an upper-level low pressure system over the US northeast this evening.

 

ISA is a standard idealized model of the atmosphere, while the real atmosphere is dynamic, and conditions can vary greatly from the ISA model at any given altitude on any given day.

You guys are killing me.

 

ISA+10 just means whatever ISA is for that altitude (which is found on the PROG Page).

 

If ISA was -55 for whatever altitude and the SAT was -66, then it would be COLDER by 10 degrees or SAT-10.

 

ISA- is good. ISA+ is bad.

You guys are killing me.

 

 

This thread could go on for a couple of more weeks about something I imagine you would know before you ever set foot on a airliner flight deck.

I only wanted to know what ISA + 10 was referring too ISA DEV or ISA at that altitude. JRBarrett explained it quite in detail thanks to all.

I believe, in this case, the + or - before the number implies deviation (dev) because an absolute temp. would simply be stated as a number without the + or -

 

Dave

The PI page that Vernon was looking at has a few columns for cruise performance, I think.

 

Here's an actual use for this discussion:

 

Let's say you're going from Anchorage, AK to Lihue, HI. You take off and are cleared directly to your filed cruise altitude of FL350. It's a bit over your optimum of FL340.

 

Now let's say as you head south, you cross a boundary into a different airmass and the ISA DEV went from -10 to +15. A big swing. The cruise performance will decrease. Hopefully your weight decreases to raise your MAX and OPT before the temp change lowers it. But, it has happened where there has been a big enough swing that planes have flown over there max altitude and had to descend. This is exceedingly rare, but it can happen.

That would impose about a 12,000 foot penalty in altitude as far as performance correct ? Compared to what you had at -10, weight being rqual of course.