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Lnav Or HDG Select On DVR8M SID?  
User currently offlineAjaaron From United Kingdom, joined Nov 2000, 123 posts, RR: 0
Posted (13 years 4 months 3 weeks 2 days 16 hours ago) and read 4579 times:

This SID requires a 180 turn to the right at 2.3 DME IWW to intercept the 082 radial to DET by DET 30.

On Boeing aircraft e.g 757/767 I have noticed that pilots take this turn in
Heading Select rather than LNAV - any reason why.

Does one not trust LNAV until its integrity is checked once airbourne for say a few minutes? In contrast to 777 and 744 - where FD is engaged in LNAV and VNAV while on ground. So is the 757/767 different, less accurate in terms of inertial reference perhaps?

If in HDG select would one select the heading to the right gradually to establish the course on the 082 inbound, (to adjust for wind) or just wind in 082.

Just a matter thats bugged me for a while.



9 replies: All unread, jump to last
User currently offlineCdfmxtech From United States of America, joined Jul 2000, 1341 posts, RR: 24
Reply 1, posted (13 years 4 months 3 weeks 2 days 15 hours ago) and read 4562 times:

In contrast to 777 and 744 - where FD is engaged in LNAV and VNAV while on ground.

Incorrect statement. LNAV & VNAV are ARMED on the ground. It is engaged when certain prereqs are met.

LNAV on the B757 and MOST Boeing requires to be within a certain range of the active waypoint in order to be engaged. I'll let one of the guys wearing stripes offer a little more.

User currently offlineAJ From Australia, joined Nov 1999, 2406 posts, RR: 24
Reply 2, posted (13 years 4 months 3 weeks 2 days 15 hours ago) and read 4549 times:

Conducting initial turns in HDG SEL is done for a few reasons.
LNAV can be slow to sequence, which delays the turn. Airports like LHR and FRA have noise sensors and the airline can be fined for flightpath deviations. The other is map updating. If it has been a while since the IRSs were aligned the map can be significantly out by takeoff unless the aircraft is fitted with GPS. LNAV is not selected until the map has updated once airborne.

User currently offline411A From United States of America, joined Nov 2001, 1826 posts, RR: 8
Reply 3, posted (13 years 4 months 3 weeks 2 days 15 hours ago) and read 4543 times:

Indeed, AJ has hit the mark.
At many airports that are extreamly noise sensitive, HDG SEL can offer superior navigation...provided the pilot(s) keep a close eye for deviation from the desired flight path.

Can you imagine...these DP's (old term SID) can actually be done with plain old VOR/NDB.

However, some that CANNOT be are so noted, and may generally require RNP 0.030...quite accurate.

User currently offlineCx flyboy From Hong Kong, joined Dec 1999, 6811 posts, RR: 55
Reply 4, posted (13 years 4 months 3 weeks 2 days 14 hours ago) and read 4548 times:

In the 777, LNAV engages as 50ft radio altitude if within 2.5nms of the active leg. (You've seriously taken off in the wrong place if at 50ft you are outside 2.5mns from the correct leg!). At 400ft, VNAV engages.

User currently offlineRick767 From United Kingdom, joined Jan 2000, 2662 posts, RR: 50
Reply 5, posted (13 years 4 months 3 weeks 2 days 11 hours ago) and read 4517 times:

We don't arm LNAV on our 757/767 a/c before departure (it is selected on the climbout if map integrity and the magenta line seem ok). I know many airlines do arm LNAV prior to takeoff though.

The initial turn on the DVR8M/LAM8M can easily be done in LNAV, but is quite a tight one at the best of times (especially at high weight on the 763) and HDG SEL gets us round the corner a bit quicker than LNAV (which can lag a bit as has been mentioned).

Since we really need to keep Flap 5 in for this first turn it can be easier to go HDG SEL and FL CH @ Vref + 40 until the turn is complete when we can go into LNAV and VNAV and start accelerating and cleanup the aircraft.

Like 411A says, (most of) these procedures were designed to work with simple VOR/DME and NDB equipment, and it doesn't harm from time to time to try out a SID (they are still called that in the UK  Big grin) using raw data without all this MAP stuff  Wow!

I used to love the smell of Jet-A in the morning...
User currently offlineAjaaron From United Kingdom, joined Nov 2000, 123 posts, RR: 0
Reply 6, posted (13 years 4 months 3 weeks 2 days 2 hours ago) and read 4503 times:

Wow - thanks for the responses, very helpful.

I get the impression then that the older 757/767 updates IRS position once airborne (assuming no GPS system in use), while the 744 updates when the TO/GA switches are pressed on t/o when still on the ground. [its neat to see the runway align with the white triangle!]. Is this also the case with 777 that on pressing TO/GA for take off the updating position takes place?

Do Airbus a/c also work in the same way in respect of taking tight turns or is their LNAV mode or whatever the equivelant is more better than Boeing.

Thank you for your responses.


User currently offlineAAR90 From United States of America, joined Jan 2000, 3589 posts, RR: 44
Reply 7, posted (13 years 4 months 3 weeks 2 days 1 hour ago) and read 4499 times:


Careful you don't get tangled in the different names of different navigation positions. Once in motion, the 757/767 IRS position is never updated. The FMS calculated position is updated by outside sources --primarily DMEs & if equipped GPS. Depending upon the airline's certification the 757/767 may not be legal to fly LNAV during departures. OTOH, 777 and 744 are newer designs that probably have newer certifications permitting LNAV engagement on departures. i.e. AA's 738 is legal to fly LNAV at all times except ILS approaches.

*NO CARRIER* -- A Naval Aviator's worst nightmare!
User currently offlineCdfmxtech From United States of America, joined Jul 2000, 1341 posts, RR: 24
Reply 8, posted (13 years 4 months 3 weeks 2 days 1 hour ago) and read 4495 times:

*This info can differ from airline to airline.
But this is how the one's I work operate:

The B757/B767/B777 update their positions once their respective takeoff modes are initiated via MCP...if GPS Nav is OFF. If GPS nav is ON, the updates are inhibited.

User currently offlineLdaops From , joined Dec 1969, posts, RR:
Reply 9, posted (13 years 4 months 1 week 5 days 21 hours ago) and read 4423 times:

Talking about B777 we prefer to TO with the HDG selected instead of LNAV because the HDG responds more quickly than LNAV.
We use LNAV during TO only when we have to perform A FMS departure that require the use of GPS or IRS.

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