Switch to TRUE from MAGNETIC - when?

[VHOJT 172]

Hi there allFlying LHR SFO soon and I was wondering when I need to switch from true to magnetic roughly - I read somewhere it was 67N?Also - completely unrelated - some texts say FL100 to descend to in decompression, otheres FL140 - which is the correct decompression descent level for a standard 744?Thanks for any help anyone can giveCheersRudy

[lostmoon 0]

hi rudy,sorry no one's answered your call. i can tell you, that i flew from KORD to VHHH, and although the switch still stayed at NORM, my PFD automatically annunciated TRU. So, i guess the queen does it on its own, but to the extent of the reality, i cannot answer.as for decompression... i have the slightest idea. i would guess 10,000 feet. sincerely,tomas

[cowpatz 243]

Hi Rudy,I think you might mean "When do I switch from MAG to TRUE?" as you will normally be in MAG. The Queen will automatically select TRUE when needed. It will change from MAG to TRUE when North of 82N or North of 70N when between 80W and 130W or South of 82S or South of 60S when between 120E and 160E. Having said that you may need to manually select TRUE when flying on some of the legs in the NCA (Nothern Canada Airspace) as some of these tracks are given in TRUE. Check on your chart.Your second question relates to the descent altitude during a decompression. The Altitude that you should initially descend to is MSA or 14000ft whichever is the higher. ETP (Equi time points) fuel planning is based on 4 hrs at 14,000 and the remainder at 10,000 ft. This ETP fuel planning needs to cover 3 scenarios. Flight from ETP to Altn with 1 eng out. Flight to altn with 2 engs out and flight to altn depressurised. Each has a different fuel policy associated with it. The one that needs the most fuel is considered when calculating the total fuel to be put on board (usually depressurised). Airline flight planners will look at suitable alternate airfields on the route to use as emergency airfields. On a typical flight from KLAX to EGLL (Heathrow) you might have CYWG (Winipeg), CYYR (Goose Bay), BIKF (Keflavik in Iceland) and EGCC (Manchester). The flight planning computer will calculate points between each enroute alternate airfields and estabish a position where if you were to suddenly depressurise you will be equal flying time to each alternate at 14000 ft for 4 hrs and the remainder at 10000. If there is no wind at these altitudes then this point on your track will have an equal distance to the airfield ahead and the one behind. So on the flight above you would have ETP points noted for KLAX/CYWG, CYWG/CYYR, CYYR/BIKF, BIKF/EGCC, EGCC/EGLL. The computer would work out, in turn, the fuel to get to each of those ETP's, depressurise and then on to one of the alternates at 14000 for 4 hrs then at 10,000ft plus of course a holding allowance at the alternate field (15 min in our case) plus approach and landing fuel. The computer looks at each of these calculated fuels and then compares this to the normal fuel needed to get from KLAX (A) to EGLL (B)plus a diversion to your destination weather altn airfield ©. If the most critical total fuel ETP figure is more than your normal A to B to C fuel then you need to load on extra gas over and above the A to B to C fuel to equal the calculated ETP fuel figure.On the flight plan (log), that the crew uses in flight, each ETP point will be noted along with the fuel figure need to ensure flight from that ETP point to your enroute altn airfield in case of cabin decompression. It is a simple matter to look at the FMC and read off the FMC predicted fuel at that point. For example if the ETP for BIKF/EGCC happened to be 40 miles before crossing 20W you could create a new waypoint 40 miles before 20w and then read off the fuel at that point (No need to EXECute as you just want to check what fuel you will have at that point). As long as this fuel is greater than that required from the flight log then you are OK. If it is less than that required then you have to look at other options (different ALTS or even diverting).Flying at 14,000 for 4 hrs with an oxygen mask on can be #### uncomfortable for all concerned so a descent to 10,000ft so as to be able to remove the mask would be desirable. There is an easy rule of thumb to use to work out if flight at 10,000 ft instead of 14,000 is possible. Take a look at the time from the ETPD (Equi time point depressurised) to the enroute alt. Lets say it is 2 hrs 30 min. Normally you would have to fly at 14,000 for most of this time. But you might find that when you look at how much actual fuel you have on board at that ETP point you actually have more than the minimum required. This excess fuel can be used to fly at 10,000 ft instead of 14,000 (Greater fuel burn at 10,000 than 14,000ft). Each 1000kg of fuel over the minmum will allow you to fly for 1 hr at 10,000ft instead of 14,000. So looking at our example if you had 1000KG extra at the ETP point you would be able to fly at 14,000 ft for 1 hr 30 min and the remaining hour at 10,000ft. If you had 2500kg then you could fly the whole way at 10,000ft. Of course once you are underway you could fine tune this with FMC predicted fuels at the alternate.Sorry the post was so long but there are some basic principles that need to be explained to give an adequate response to your question.CheersSteve

[VHOJT 172]

Thanks so much for your fantastic answer Steve, and thank-you also Tomas for your reply! Every question I had has now been very well answered. There is certainly a lot of interesting/useful information in your response, you should perhaps consider putting it on PMDG Ops. Thanks again!My flight planner, FOC 2003 calculates these ETPs in quite a lot of detail as you have mentioned - I may have a look at the altitudes though. My only two questions that would arise fromy our response are these:Does your dispatch add additional fuel for icing (or anti-icing really)?Do you try to select enroute alternates with ILS, even if the predicted weather is fine for a visual?Don't feel you have to reply to these, I'm pretty sure I know the answers already.Again, thanks for your great reply, Steve.Rudy

[markadeane 272]

Good explanation Steve, thanks!Can anyone comment on whether or not the PMDG 744 models the ETP display? I haven't been able to nut it out myself.I use the fix page and some basic maths to display my ETP's

[VHOJT 172]

MarkBy what I know and from what I have seen on the Virgin 744 DVD, and as Steve said, "they", the pilots, get the equi-distant point from dispatch on the flight plan between two airfields in form of a distance from them, (and also perhaps an ETP in terms of a distance from a waypoint along the route). This allows you to draw a radius of this distance around the airfield on your ND, and the other airfield too. Somewhere, the circles will intersect, or abeam on your route to where they intersect will be the ETP. I think it is also given in the form of a distance from a waypoint enroute, so you can also draw a circle aroudn this too if you wanted.Here is an example from a recent flight of mine:This this the bit of the flight plan that tells me the ETP:ETP: ADINA 0030 10.09 121994 49223 272012 43.68N/031.14E STOPS RET: LTAC 0230 00.27 5242 24295 266770 NCR/210/000 S S FWD: LROP 0229 00.27 5222 24275 266790 NCR/210/000 S S Sorry it's a bit out of alignment. The first line tells me the equal time point on my flight plan, so ADINIA 0030 means 30nm from ADINA is the point, it then goes ont to tell me fuels and times and weights. Next line down tells me the distances and times for the two airports I could go to.The first one, which I could RETurn to: Ankara, Turkey (LTAC), is 230nm away at the ETP, or 27 minutes at FL210 (probably shouldn't be FL210). It goes on to tell me fuel required and weights and stuff like that.To go ForWarD: LROP (Bucharest) is 229nm away and 27minutes. You can draw arcs around these two airports using those distances on the fix page, and you get what you see below (hope you can see it okay on the ND), sorry it's not too obvious, you can see the circle of LTAC intersecting that of LROP further up on the ND along the route:http://forums.avsim.net/user_files/151162.jpgI may be a bit off here, so feel free to correct me anyone who knows better!!CheersRUdy

[SAS263 1]

Mark, to the best of my knowledge you

[VHOJT 172]

Hi againYou have an interesting idea there Tord, I'll give it a goCheersRudy

[cowpatz 243]

The easiest way to dispaly ETP's on the ND is to have them as a time fix. For example if your ETP point is 40 miles prior to XXXXX waypoint then create a waypoint there but do not EXECute it. Select RTE Data and take note of the time at that waypoint. Press the ERASE key. Go to the fix page and enter the time in the bottom RH section of the FIX page where it says PRED ETA . ALT. This needs to entered in the 24 hr clock format with a z after it ie 1245z. This will show up on your ND as a small green donut with the time next to it. This is approx your ETP point. A simple ETP point can be found in three ways inflight.If you are on a charted route and both enroute alternates (lets assume both are off our track) show on it then simply fold the chart so that both alternates are over the top of one another. A bit tricky but can be done. Squash down the folded edge to form a crease line. Open the chart and see where the crease line crosses your track. This is the still air ETP point. Take a stab (or measure) the distance to the next waypoint. Enter this as a pilot created waypoint ie XXXXX/-45 would be 45 miles short of XXXXX. On the legs page type in the altn airfield under this created waypoint. Note the distance. Type in the other Altn airfield and note the distance. Press the erase key. Now compare the two distances. Adjust the position of the ETP until both distances are in agreement. The final point is your still air ETP point. Now have a look at your MET info to get the winds at 10,000 to 14,000 ft and adjust the ETP point accordingly. It moves into the wind. This is the final position of the ETP. It might seem cumbersome but can be done very quickly with practice.The other method is for when one or both Altns are off the chart.Hope this helps.CheersSteveIn our airline ETP points are always shown as a distance before a waypoint ie XXXXX/-231. This way when we are flying along track and XXXXX becomes the active waypoint we can see when we have 231 miles to run to it and know we have crossed the ETP point.Skip the chart folding exercise and go straight to the legs page. Take an educated guess as to which waypoint would be closest to the ETP. Type in the Altn airfield below this waypoint and note the distance. Do the same with the other and note the difference in the distances. Create a waypoint using XXXXX/NMs or XXXXX/-NMs depending which side of the waypoint the differences suggests you go. Try it again and keep doing to there is very little difference. Once again this is your still air ETP. Use the wind info as before to fine tune.The last method is for if your Altns are close by. Put each altn airfield in the fix page as a fix. Select a suitable range circle to put in ie /300. The maximum you can put in here is 511 nms. Have a look on your ND at the range circles. Hopefully they will overlap. Imagine a line running between the 2 points where the two circles overlap. Extend this line out until it crosses your track. This is your ETP point. Create a waypoint and see how close it gets. Do not worry about Insufficient fuel messages. All will return to normal when you select the erase key. There are other ways but these ones are simple enough to understand.

[VHOJT 172]

Hi there SteveThanks again for the reply. Do you know if the PRED ETA part of the fix page is modelled on the PMDG plane? I can't seem to find it - I ahve seen a photo of the real one but it doesn't seem to be there in PMDG?CheersRudy

[cowpatz 243]

Just checked and unfortunately this does not appear to be modelled on the sim. If you think it is important then just create a waypoint at that position or write it down on a piece of sticky "post it" paper and stick it on the side of the screen. Creating a waypoint would not be done in the real world as this would interfere with the CPDLC or ADS automatic position reports given to ATC. CheersSteve