On the DES FORECAST page there is the ability to enter airfield temp with respect to ISA. Is this temperature in relation to 15C, or corrected for altitude?

 

For example; an airfield is at an altitude of 2,000ft above msl with a temperature of 10C. Is the correct temp to enter -5, or -1?

 

Also, how is the planned engine anti ice level determined? Other than looking out the window I'm not sure how you'd judge it!

From tutorial 2 it is recommended to use the ISA DEV based at FL180 due to FSX limitations. Use airfield QNH.

 

As for the anti-ice local METAR/TAF to determine cloud cover and temps.

On the DES FORECAST page there is the ability to enter airfield temp with respect to ISA. Is this temperature in relation to 15C, or corrected for altitude?

 

ISA infers that you're using whatever temperature is appropriate for that altitude. ISA at DEN is 4. Remember standards are based off of 15 degrees and 29.92/1013 at sea level. As far as what you're using, I'd use the ISA DEV at the transition level - whatever that happens to be for the area.

 

If you'd like to skip the math/calculators, just use this: http://aviationweather.gov/adds/winds/

If you're in the States, use the drop down to select FL180, and then click the radio select button (circle button) that says Temperature Difference. That's your FL180 ISA DEV.

 

Also, how is the planned engine anti ice level determined? Other than looking out the window I'm not sure how you'd judge it!

 

Water + cold = ice

Look at the wind/temps charts to find where you're in the high-risk icing temps (varies by airline SOP, but I use -40 to +10C). There's your cold.

Now, check to see if there are any cloud layers between your T/D and the field. There's your water.

 

If your cold starts at -40 at cruise and goes down to 8C at the destination, your cold is the whole descent.

Say there are two cloud layers - one at FL220 and one at FL150. From there, you just tell the FMC, there's a chance the A/I systems will be on from FL220 to FL150. That doesn't mean you can't turn it off in between, but since the A/I means you're going to use a higher idle thrust, it needs to compensate for the change in the descent as a result.

use the ISA DEV at the transition level - whatever that happens to be for the area.

 

Why transition level?

 

If your cold starts at -40 at cruise and goes down to 8C at the destination, your cold is the whole descent.

 

That'll be OAT not SAT, so that's not quite right. Thanks anyways though.

Why transition level?

The whole point of providing the QNH is to aid in the forecast. You're up running standard pressure (29.92/1013) all of cruise, and at some point you have to transition to the local setting. In order to make this 'jump' easier, the calcs in the background descend differently in order to be at the transition altitude using the destination pressure. Not a huge deal if you're going from 29.92 to 29.93, but it makes a big difference when you're going from 29.92 to 30.10+ and 29.70-. It would follow that since you're correcting for pressure, that you also correct for temperature. Because that part of the forecast is aimed at easing the transition from standard to local at transition level, I take the ISA DEV at FL180 (in the States).

 

That'll be OAT not SAT, so that's not quite right. Thanks anyways though.

 

OAT is SAT...

"In aviation terminology, the outside air temperature (OAT) or static air temperature (SAT) refers to the temperature of the air around an aircraft, but unaffected by the passage of the aircraft through it."

 

Your ISA DEV is your ISA DEV. In the graphic I provided, all you need to do is take the value depicted. Otherwise, it would have no use being on aviationweather.gov...

The whole point of providing the QNH is to aid in the forecast. You're up running standard pressure (29.92/1013) all of cruise, and at some point you have to transition to the local setting. In order to make this 'jump' easier, the calcs in the background descend differently in order to be at the transition altitude using the destination pressure. Not a huge deal if you're going from 29.92 to 29.93, but it makes a big difference when you're going from 29.92 to 30.10+ and 29.70-. It would follow that since you're correcting for pressure, that you also correct for temperature. Because that part of the forecast is aimed at easing the transition from standard to local at transition level, I take the ISA DEV at FL180 (in the States).

 

Ok makes sense, thanks.

 

OAT is SAT...

"In aviation terminology, the outside air temperature (OAT) or static air temperature (SAT) refers to the temperature of the air around an aircraft, but unaffected by the passage of the aircraft through it."

 

Your ISA DEV is your ISA DEV. In the graphic I provided, all you need to do is take the value depicted. Otherwise, it would have no use being on aviationweather.gov...

 

My mistake, meant TAT not SAT, which would likely be higher than 8 in your example.

Ok makes sense, thanks.

 

Welcome.

 

My mistake, meant TAT not SAT, which would likely be higher than 8 in your example.

 

I don't understand what you're getting at and why it matters. I was giving you a very broad example where you'd encounter icing conditions. 8C on the ground (SAT) is really inconsequential because the effect of Mach No that close to the ground is negligible. By my calc, 8C on the ground, with a 250 kt airspeed gives you about 8.1C degrees TAT. Still below 10C.

 

In any case, the ISA DEV is given for temp/pressure adjustments through transition level, so the question of what kind of temp it is, is irrelevant.

Just to clarify one minor pont regarding the ISA DEV entry on the DES FORECASTS Page...the ISA entry there is the AVERAGE ISA Deviation for the whole descent from CRZ ALT to the TDZE.

 

We have seen some rapid advances in FS WX Engine capability & the SAT is now modelled pretty accurately by the likes of AS2012 with the latest beta patches & you can get a pretty accurate number by figuring counting the numbers & doing a quick sum with your Mk1 Pencil & Paper.

you can get a pretty accurate number by figuring counting the numbers & doing a quick sum with your Mk1 Pencil & Paper.

 

That, combined with the line in your signature made my morning. Thanks Steve.

Why transition level?

 

Jordan

 

See NGX Tutorial No.2 right at the very end. It's 18000 ft not transition level. It has to do with the way FSX erractically handles wind and temperature.

 

Michael Cubine

Hy, concerning ISA DEV, a real 737 pilot, during a flight to Canarias Island has confirmed, with a T° of 21 on ground: Put +6, no matter is the QNH, it's good for the computer...

 

Just my part...

 

And apologise for my bad english, not my native...

 

Merry Xmass to all fellows NGX Fan

 

Jean-Michel Richard

No offense, but that makes no sense.

 

It's true that the ISA DEV in the case of 21 on the ground would be +6, but it being "good for the computer" doesn't really make a lot of sense.

No offense, but that makes no sense.

 

It's true that the ISA DEV in the case of 21 on the ground would be +6, but it being "good for the computer" doesn't really make a lot of sense.

It builds character.

It builds character.

 

My blunt questions, or feeding the computer friendly data? Haha...

Just a think... You an I are just Computers pilots on NGX... Not the friend, he's REAL pilot on 737NG... Then...

 

Another think, as, on this flight, we've know a full VMC, he said: Full manual under FL100 then the computer, forgot, PILOT THE PLANE and flight as you've maked at your beguining, (in french: TRICOTTE - with VOR, NDB, ADF, ILS), as I make since..... ;o). And really, a big flight, without computers (No, in fact, this is in backup, but). Then, don't forget your beguinings.

 

Happy XMass

Jean-Michel Richard