Hi,I have two questions not directly related to NGX, but I think this can be useful to other NGX pilots.1) While descending, as I cross TL (transition level) I click STD in the inner BARO knob (after setting the local QNH). This way I go from standard pressure (29.92 or 1013) to local pressure (QNH). Very often this results in the altimeter switching to a totally different altitude than the one I was flying in VNAV, sometimes 600 ft below VNAV. The airplane doesn't seem to do anything about it and continues flying above the new path altitude. Then I use V/S and enter a steeper vertical descend speed. As I get close to the right altitude I click on VNAV button and the plane intersects the vertical path and continues flying the proper path. Is this the right way to do it?2) Since I don't know how to calculate the TL I usually add 1000 ft to the TA. For instance if TA is 7000 I use FL080 as TL. Do you know how to calculate the right TL when you know the TA and the local QNH?Thank you!

what i always do on that situation, firs, wait until you below 10000ft, usually this is the altitude when the controller says QNH "1027" or something if it continues, disengaged the vnav and then engage it again, make sure, you picked a route for arrival, i mean like if your landing at gatwick london at the WILO3B 08R, make sure your in that course on LNAV. with all that mess, i hope you understood what i mean HAHA, sorry and hope it helped, Daniel

Hi,For example on the LSGS aproach chart the TA=16000 and the TL is given by ATC...Regards,Richard Portier

Generally, I only click the STD button while I'm ascending/descending. For instance, TL is always 18000 in the US. When I'm cleared from say FL250 to 15000, I'll click the STD button as I'm descending through 18000. That way, VNAV can compensate during the descent. I do understand what you are saying in that as you cross the Trans Alt, you switch to QNH and the altimeter jumps to the new pressure altitude. However, are you saying VNAV isn't making any adjustments to your descent to recapture the path?If you select STD while you are descending, VNAV will adjust the vertical speed to capture the correct path and also level off at the correct altitude based on the new altimeter setting. If you are already flying level and VNAV is engaged, selecting STD should cause the aircraft to climb or descend (in VNAV SPD I think) based on the new altimeter setting and what you want your pressure altitude to be.Does this help at all?? And maybe I'm not understanding something, but I thought transition was transition and you can reset your altimeter to QNH when descending or STD when climbing whenever you want as long as you level off at the correct altitude/flight level.

I don´t have a problem with Vnav when I set the local altimeter, even if there´s a big pressure differencial from standard. In all my flights Vnav was smart enough to adjust the vertical path after switching to local. There is a rule for knowing what the TL is going to be. In Brazil, TL is going to be TA plus or minus at least 500' depending on the difference between local altimeter setting and standard. I don´t know it by memory because ATC is responsible for giving that to you. I´ll try to dig up the rule of thumb from my private pilot textbooks

Thank you for fast replies!Actually I didn't have any issues with LNAV not readjusting vertical speed in order to intercept the new path after switching to local. This is something that has been happening for the last flights. I am not sure it starting happening right after SP1. I would say that it worked well after SP1, but suddenly something seems to have gone wrong. I haven't made any changes to ASE, not that I can recall.

I just did one more test flight and LNAV keeps refusing to intercept the new path. After a couple of minutes I get the message DES PATH UNACHIEVABLE.Now I am too tired to think anymore. I will ask my pillow for advise...

Generally, I only click the STD button while I'm ascending/descending. For instance, TL is always 18000 in the US. When I'm cleared from say FL250 to 15000, I'll click the STD button as I'm descending through 18000. That way, VNAV can compensate during the descent. I do understand what you are saying in that as you cross the Trans Alt, you switch to QNH and the altimeter jumps to the new pressure altitude. However, are you saying VNAV isn't making any adjustments to your descent to recapture the path?

Yes, that is exactly what I mean.

I don´t have a problem with Vnav when I set the local altimeter, even if there´s a big pressure differencial from standard. In all my flights Vnav was smart enough to adjust the vertical path after switching to local.There is a rule for knowing what the TL is going to be. In Brazil, TL is going to be TA plus or minus at least 500' depending on the difference between local altimeter setting and standard. I don´t know it by memory because ATC is responsible for giving that to you. I´ll try to dig up the rule of thumb from my private pilot textbooks

My NGX obviously has a lazy VNAV.I have searched the net trying to find a way to determine TL out of QNH and TA. It seems to be a well kept secret.

There is also a place to input QNH on the descent forecast page. That said, I don't know how, if any, affect this has on the FMCs planning. Anyone know?

There is also a place to input QNH on the descent forecast page. That said, I don't know how, if any, affect this has on the FMCs planning. Anyone know?

Oh that was interesting. I will have to check that once I get home.

There is also a place to input QNH on the descent forecast page. That said, I don't know how, if any, affect this has on the FMCs planning. Anyone know?

FMC, descent forecast. On the right you can enter qnh en temperature. But i think it is only for cabin pressure.

I don´t have a problem with Vnav when I set the local altimeter, even if there´s a big pressure differencial from standard. In all my flights Vnav was smart enough to adjust the vertical path after switching to local.

I would say the same here, variations never are so huge therefore at least for me the VNAV can somehow compensate the transition with no big trouble.

I would say the same here, variations never are so huge therefore at least for me the VNAV can somehow compensate the transition with no big trouble.

In principal no, but certainly enough to miss an altitude constraint.I've looked at the FCOM, but I've not managed to find anything that says what the QNH info on the DES FORECAST page is actually used for.

Straight from the IVAO Academy:

Calculating TRL for any TA

QNH less than 1013STEP 1: STD QNH (1013,2 hPA) MINUS local QNH = "X"STEP 2: MULTIPLY "X" with 28ft = "Y"STEP 3: ADD "Y" to TA (Transition Altitude) = "Z"STEP 4: select the FIRST FL that is GREATER than "Z"You have now calculated the nearest FL above the TA that can serve as a TRL.Depending local regulations, you may have to ADD 1000ft to ensure vertical separation to the level you calculated in the line above.Don't forget to put the TRL in the ATIS!Example: TA 6000ft and local QNH 973STEP 1: 1013.2 hPa - 973 hPa = 40.2STEP 2: 40.2 x 28ft = 1125.6 ftSTEP 3: 6000 ft + 1125.6ft = 7125.2ftSTEP 4: first FL greater than 7125.2 ft = FL75FL75 can be selected as TRL, but there will not be 1000ft vertical separation with aircraft at the TA (only 374.8ft).If 1000ft vertical separation is need between TA and TRL, FL85 has to be selected as a TRL (FL80 is only 874.8ft above TA). QNH 1013Since the QNH in the METAR is always rounded down to the nearest whole hPa, ADD 500 ft to the TA to obtain the TRL.To have 1000ft separation between TA and TRL (vertical separation between TA and TRL): ADD 1000ft to the TA QNH more than 1013STEP 1: local QNH MINUS STD QNH (1013,2 hPA) = "X"STEP 2: MULTIPLY "X" with 28ft = "Y"STEP 3: SUBSTRACT "Y" from TA (Transition Altitude) = "Z"STEP 4: select the FIRST FL that is GREATER than "Z"You have now calculated the nearest FL above the TA that can serve as a TRL.Depending local regulations, you may have to ADD 500ft or 1000ft to ensure vertical separation to the level you calculated in the line above.Don't forget to put the TRL in the ATIS!Example: TA 6000ft and local QNH 1043STEP 1: 1043 hPa - 1013.2 hPa = 29.8STEP 2: 29.8 x 28ft = 834.4 ftSTEP 3: 6000ft - 834.4ft = 5156.6ftSTEP 4: first FL greater than 5156.6 ft = FL55FL55 can be selected as TRL, but there will not be 1000ft vertical separation with aircraft at the TA (only 343.4ft).If 1000ft vertical separation is need between TA and TRL, FL65 has to be selected as a TRL (FL60 is only 843.4ft above TA).