I will be starting few topics to answer some of the questions that are currently bugging me a little bit. They may look the same at first, but they are not. I just don't want to ask five different questions in the same thread. 

 

Now, some of you may remember that few months back I started flying NGX and I had a few silly questions at that time. I',, glad to say that few hundred hours later, I'm fairly confident in my abilities to handle this virtual bird. That, of course, means I'm ready to consider the things that I a) paid no attention to and b) was doing wrong. here comes one.

 

Please note: I am not a real-life pilot, I am not a real-life student pilot (wife does not approve the expenses yet, and I'm not that stupid to question her decisions at this point - learned that lesson). All I know comes from books (that I love to read), FSX flying experience and online articles/discussion forums. Treat me gently.

 

Transition Altitude Barometer Settings

Now, I vaguely understand the concept of the transition altitude (18000 in US, whatever the chart says elsewhere) and its relation to the altitude barometer settings. I've read weather chapters from FAA's "Pilot's Handbook of Aeronautical Knowledge" as well as few other places, but my question still remains unanswered, so I decided to tap into the collective brain trust on this forum.

 

In many cases when I descend at the transition altitude, or thereabouts, I change the altimeter setting from the standard 29.92/1013 to the one of the destination airport. If this setting is significantly different, altitude "jumps" or "drops" and this causes the engaged VNAV to try to correct, causing some (perceived?) instability. I don't like that and I'm pretty sure I'm doing something wrong. 

 

From what I understand, I need to do some forecasting of the sort to prevent this sudden change from happening and what I believe, this has something to do with the "FORECAST" sub-page page of the "DES" page in the FMC. However, my (admittedly, quick) research on this topic caused more pain than gain.

 

Any suggestions how to handle this transition? Please, keep in mind that pointers to books/articles/etc. are more than welcome.

 

I'm trying to understand what's happening and not necessarily looking for "a recipe".

 

Thans everyone in advance.

 

 

Any suggestions how to handle this transition? Please, keep in mind that pointers to books/articles/etc. are more than welcome.

 

The jump is inevitable, but using the FORECAST page tells the system to expect it.  Enter the destination altimeter setting on the FORECAST page well before the T/D to give it time to adjust the T/D accordingly.  Additionally, set your altimeter to the local setting as soon as the controller clears you below the transition level to give the system more time to react to the jump (if necessary - particularly when you forget to enter the info on the FORECAST page).

I'm pre-setting the pressure on the altimeter knob to the destination airport as soon as I'm reasonably certain no new METAR will come from that airport before I arrive. I have not been setting it in the FORECAST page and I believe this may be what I was doing wrong.

 

I do have follow-up questions.

 

1) Is setting the pressure to the destination airport correct procedure, or should I use the information from the closest source (nearby airport)?

 

2) This is a whole new can of worms, I suspect: Does temperature play role here? If say destination airport is at 28.85, do I have to adjust this value somehow based on the temperature and the altitude? 

 

Thanks.

 

 

1) Is setting the pressure to the destination airport correct procedure, or should I use the information from the closest source (nearby airport)?

 

Use the destination airport.  While in GA, it's best to use the closest station, in IFR flight, the controller is going to give everyone the destination field's setting to keep everyone on a similar setting.  Entering the destination altimeter (both preset in the ALT setting, and on the FORECAST page) is just anticipating what the controller will be giving you, eliminating potential variance.

 

 

 

2) This is a whole new can of worms, I suspect: Does temperature play role here? If say destination airport is at 28.85, do I have to adjust this value somehow based on the temperature and the altitude? 

 

It does.  This is why there's an ISA DEV (international standard atmosphere deviation - the deviation from the "normal" temperature lapse rate as altitude increases) entry on the FORECAST page.  I think the manual mentions something about using the ISA DEV at the transition level, but I just use the destination air field.

 

Here's how to find the value, though:

http://aviationweather.gov/adds/winds/

^^ Go here

Click on the little button for Temperature Difference (above the chart)

Adjust the altitude to FL180

Find your approximate position

Enter that value in the ISA DEV field on the FORECAST page

ISA DEV and pressure entries on FORECAST page have no effect on VNAV path for now. This would be modelled in SP2.

 

In real world cold temperature corrections are specially made for approaches (cold weather may inflate altimeter readout, so it could be dangerous), but it's not done in FSX (as temperature doesn't effect altimeter in sim).

Side note,

 

 

 

In real world cold temperature corrections are specially made for approaches (cold weather may inflate altimeter readout, so it could be dangerous),

 

if you ever run into a procedure that says "prohibited under -20C" or something to that effect, this is why.

Especially on BARO VNAV/RNP procedures.

Use the destination airport. While in GA, it's best to use the closest station, in IFR flight, the controller is going to give everyone the destination field's setting to keep everyone on a similar setting. Entering the destination altimeter (both preset in the ALT setting, and on the FORECAST page) is just anticipating what the controller will be giving you, eliminating potential variance.

 

 

It does. This is why there's an ISA DEV (international standard atmosphere deviation - the deviation from the "normal" temperature lapse rate as altitude increases) entry on the FORECAST page. I think the manual mentions something about using the ISA DEV at the transition level, but I just use the destination air field.

 

Here's how to find the value, though:

http://aviationweather.gov/adds/winds/

^^ Go here

Click on the little button for Temperature Difference (above the chart)

Adjust the altitude to FL180

Find your approximate position

Enter that value in the ISA DEV field on the FORECAST page

Just to elaborate a bit on point 2, while the pressure does depend on the temperature (if you remember the gas laws from physics: higher temperature => higher pressure) this is already included in the value reported in the metar / atis, so if that says 29.85 you don't have to adjust that value yourself.

As Kyle points out, your plane's performance will depend on both the temperature and the pressure and thus the DES forecast page lets you enter both.

 

 

In many cases when I descend at the transition altitude, or thereabouts, I change the altimeter setting from the standard 29.92/1013 to the one of the destination airport. If this setting is significantly different, altitude "jumps" or "drops" and this causes the engaged VNAV to try to correct, causing some (perceived?) instability. I don't like that and I'm pretty sure I'm doing something wrong.

Most of your other questions have been addressed, but the perceived instability can be handled in the same you you make most changes in aviation. In stages. If the destination pressure is significantly different, then instead of one big change, make many smaller changes.

Thank you everyone for fantastic responses! I knew I could rely on the brain trust here :-).

 

So, to recap, here's my readback.

 

Jump of the speed tape is expected and normal, but Paul offers the workaround to change to the destination altitude gradually. Providing the destination altimeter setting into the FORECAST page will prevent the plane in VNAV from jumping when the transition happens, as the FMC knows what's going to happen. The temperature does play a role IRL, however this has not yet been modeled in the NGX.

 

Readback correct?

 

 

Readback correct?

 

Readback correct.