Hi simmers and B767 experts out there!I have a question regarding the real FMC "direct to" function:When entering a waypoint to fly direct to (LSK 1L), will the real 767 turn towards this waypoint and fly direct to it or does the FMC calculate the track between the aircraft's present position (when entering the "direct to") and the new waypoint and intercept this track?Example: I am overhead present position A and enter "direct to B", which needs a right turn by 60 degrees. Does the real plane turn right towards B or does it turn right to intercept the calculated track between A and B?I noticed that PIC767 intercepts the track between A and B, so especially during large turns it "overshoots" waypoint B initially in order to intercept the calculated track. How does the real thing behave?Any help would be highly appreciated!Best Regards,Philip (LOWW)

Philip,Good question.In real life it actually turns to intercept the course it has drawn from the point in space when you executed the "direct to B" operation. One of the things that left me a bit dissappointed with the 767 PIC program is the "S turns" the aircraft will make to get on course after executing a "direct to" operation. The 757 generally does a very good job of rolling out right on course (as a form of technique, if the turn is more than say, 45 degrees or so, we will ask the PNF to re-execute the "direct to" a second time as we near the course heading...this is to minimize any "S turning"....give it a try :)).take care,BBall------------------ Capt. William "BBall" BallBoeing 757, Northwest AirlinesSenior Editorwww.frugalsworld.com

Hi BBall!>>In real life it actually turns to intercept the course it has>>drawn from the point in space when you executed the "direct to>>B" operation.Thanks a lot for your reply! So it seems that PIC767 is handling this correctly! ;-) Do you know why the FMC is programmed this way? Seems a bit illogical to me, because when I enter a "direct to B" I intend to fly to B on the shortest way and do not really want to intercept the calculated track...>>we will ask the PNF to re-execute the "direct to" a second time>>as we near the course heading...this is to minimize any "S>>turning"....give it a tryYes, that's exactly what I do during large turns, but I was wondering how the real plane handles this. Good to know that in real life it works this way as well... :-)Many thanks again!Best Regards,Philip (LOWW)

You can quickly overcome this by double clicking on LSK1L again to re-establish the more direct route to the waypoint in LSK1L.CheersIan

Hi William, maybe I'm just misreading your answer or maybe our 757's do it a little different than yours. When on a heading of, say, 360, and you do a DIRECT TO a waypoint that requires a 45 degree turn to the right, the FMC will create an arc using the turning rate (dependent on groundspeed and bank angle) and from the end of the arc, create a direct track to that waypoint. NOT a direct track from the point in space where you executed the DIRECT TO. You can see the curve (arc) on the EHSI and from there a straight line. So there's no intercepting, it's just an LNAV track. In practice, the airplane follows the track perfectly, so no need to re-execute the DIRECT TO. The only times when you do need to re-execute is for instance when you're accelerating and the arc is calculated with the speed you had when you executed. So your turn will actually be wider and you end up outside the arc. This occurs with deceleration as well, of course.So William, I'm wondering if your 757's actually do it differently than ours or this is exactly what you meant :)Our 757's have all the avionics upgrades (PIP's - product improvement packages). It's not a Pegasus package, but we do have the new EADI with speed tape, however no A/T annunciation as we see in 767PIC. The FMC is also upgraded, it's newer than the Pegasus version (have worked with Pegasus package in various simulators). Our autopilots have all the functions of the 747-400 autopilot (it's exactly the same thing) with pushable altitude selector, for example. So who knows, maybe there is a difference here. I always thought that the direct to function worked really crappy in 767PIC :)Iz

Hi Iz,thanks for your reply! Funny...so is the PIC767 working correctly or not? Or do the various FMC software versions handle this function differently?Well, your description seems a lot more logical to me...when I enter a DIRECT TO I intend to fly to this point on the shortest way and do not want to intercept any calculated track.Thanks a lot for your help!Phil (LOWW)

Hey Iz and Phil,This one really got me thinking after I gave my above answer. I flew two (apx) 900 nm legs today, and watched closely exactly how the FMC was handling our "direct" clearances. :)Let me say Iz, that all our 757 FMCs are operating with a "Pegasus" upgrade package. Here is what I observered....and it sort of shocked me.It handled them BOTH ways!At each ATC "direct" clearance, I turned the "range" knob down to the 10 nm scale and observed what happened. About one third of the time (all times in level, constant speed flight) the HSI would show that the FMC calculated a "track" from the point in space that the direct was executed, then turn to intercept that track. This of course, caused the "S turn" manuver I mentioned above. The other 2/3rds of the time, it would calculate a track from the nose of the aircraft to the intended waypoint and turn only as far as a "direct routing" would require to head toward that waypoint.Interesting, eh?I didn't see any correlation to amount of turn required, etc. In fact, one of the times it did the "S turn" manuever, we were on an ATC heading of apx 290 degrees, and we had to turn to an apx north heading to go "direct" to the next waypoint (in other words, probably our biggest turn all day when using the LNAV feature).I plan on trying the following little "flight" in 767 PIC when I get home from this trip. ***Put yourself on runway 22L in KBOS. The flight plan should be KBOS dir MHT dir CAM dir SYR J547 BUF etc, etc (the normal routing for our flights westbound out of Boston). The Logan 2 SID calls for a heading of 140 degrees after departure. Take off, at 500' agl, turn to 140 degrees. At about this "part in the movie", BOS Tower hands you off to BOS Departure Control. They will give you a heading of apx 090 degrees and climb you to 12000'. Roughly through 5000', they'll give you clearance to turn NE bound (040-030 something like that)....within a few seconds, they'll give you this clearance...."XYZ 123, cleared direct CAM then flight planned route, contact Boston Center on....". Line select CAM to the number one position on the LEGS page, execute and engage LNAV. Of the many, many times I've flown this departure routing in real life, I can not remember a time when it didn't draw the line from a "point in space" thus setting yourself up for the "S turn" manuever and/or the "reselecting direct" dance.Try it and see what it does.....I pretty much guarantee that "767 PIC" will do the "S turn" thing. So based on what I've seen our 757s do lots of times, and from what I saw occur several times today, I would say that "767 PIC" is accurate with regards to how the FMC handles "direct" clearances....well, at least 30 % of the time...LOL. fun topic.:)later guys,BBall----------------- Capt. William "BBall" BallBoeing 757, Northwest AirlinesSenior Editorwww.frugalsworld.com

William,I recall an old post from Eric on this same subject matter. He also mentioned that the AA '67 tracks a DIRECT TO segment from the point of engagement to the target waypoint, and that "S-turning" was not uncommon. He mentioned for turns > 90 degrees most pilots would often use HDG SEL to get the aircraft turning and pointed in the general direction of the waypoint and then execute a DIRECT TO - that or they do the "double" DIRECT TO technique you mentioned in your earlier post.Anyway, I think PIC is correctly modelled after the AA '67 which was used as its baseline. It is possible that this behavior is a customizable carrier feature, I s'pose.Personally, I wished it worked the other way (i.e. the aircraft tracked directly to the waypoint w/o regard for the point at which it was engaged). If I wanted to track an inbound course, isn't that what I have the INTC option for?J

jase439,I guess I was a bit shocked the first time I saw this "S turn" thing in the simulator back in '97 during my initial check out on this bird. One would've guessed that something this sophisticated wouldn't require the pilot to have to reselect the "direct" function several times when makeing a big turn. In real life, I tend to do just exactly as you mentioned....I use HDG SEL to get myself pointed in the direction of the waypoint, then have the PNF execute the "direct" operation, then engage LNAV....:).You'll see in my review of "normal operations" that I penned for 767 PIC (to be published at Frugalsworld soon); the incessant "S turns" were one of the niggling items that I was less than impressed with. Yeah, the real aircraft will do it, but not nearly as much (or often....read above post), or as pronounced as the sim software.later,BBall------------------Capt. William "BBall" BallBoeing 757, Northwest AirlinesSenior Editorwww.frugalsworld.com

William, Jase, interesting. Finally flew again yesterday after a months' vacation (Amsterdam to Banjul, Gambia, and back! 11:42 hours block time!). I guess it's a PIP function then. I have never seen the S-turning, and as Jase says, if you really want to intercept a given track to a waypoint, you can use the INTC option in the lower right portion of the CDU screen. A sophisticated plane shouldn't have you intervene with HDG SEL to get it going :) Like I said, as long as the airspeed remains the same during the turn, it'll follow 180 degree turns or more on LNAV perfectly.I find the behaviour as found in 767PIC quite a nuisance. But maybe I'm just spoiled :)Iz

Hey B-Ball maybe your airline has not updated its FMC software lately -"LOL" There are so many different FMC variations and software change's and service bulletins incorporated into individual airlines FMC's there is bound to be some odd behaviors occasionally.For a VR (heading to a radial) to a CF (course to a fix) leg change, the FMC may not make a proper transition. Instead, the VR leg may contain an extension which goes beyond the transition point. The airplane may follow this extension beyond the transition point before switching legs. The cause of this anomaly is due to VR legs erroneously using the FMC computed magnetic variation for route construction rather than the magnetic declination associated with the navaid.Default Intercept-Course-To Leg Skewed From Original Leg -Under certain conditions, when using the default course on an

As Eric mentioned in a related post: PIC seems to model the best and worst (whether by design or just by oversight :) aspects of the real aircraft.It seems silly that a cockpit crammed with precision technology would require the pilot to "coerce" it into making the correct choice ;-) But then: it's only software!I worked a contract for a major aviation company a few years ago working on next-gen GPS-based landing system to "succeed" existing ILS equipment. The tolerance of the landing system was on the order of centimeters plus it featured high tech encryption/encoding of the signal itself. There were all kinds of a fail-safes, internal calibrations, corrections for detecting bad GPS data, etc. Ironically, with all the technology that was packed into this thing, it had a simple RS-232 port w/ modem for remote maintenance purposes. If you had access to the dial-in number, you could pick up a telephone and "whistle dixie" at the modem carrier and take the landing system off the air :-eek (fortunately, I wasn't responsible for that code!)J

I know what your saying Jase, just look at how the FMC nav database's are uploaded to the FMC's ,with floppy's of all things.Im prepping a china eastern 737-700 for delivery tommorow and just installed the jep. navdata base with 6 floppys, talk about stone age.

Isn't it funny that after 20 years of technological advancement we still have boat-anchor technology like the "floppy drive" still running about ;-)J

Here's one I prepared earlier. Just made a "direct to" action over Turkey. We're almost rolled out on the new heading actually, I took me some time to get the camera out after executing :) You can see where the track line behind us begins, that's where I executed it. Then a smooth turn to a new track towards the desired waypoint.Iz