VNAV Speed?
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VNAV Speed?
Hi Guys!!!
I have a couple of doubts and I wanted know if you can give me a hand.
Well, let's begin... The plane: 734, FMC:10.7, destination: SKBO.
we were instructed to proceed BUV, VULAM, BOG. We discussed about "what if" they lead us off the lateral path with Radar Vectors, how will we do the descent? VNAV Speed, FL Change or V/S... Well, we choose VNAV Speed. When we received the first radar vector, I couldn't select the VNAV speed prompt, the option wasn't there. The other option was pressing the Speed Intervention... So, I did, but the plane remains in VNAV path, even changing the Speed. The radar vector was 15 miles before VULAM aprox, so I Was flying with a heading betwen VULAM And BOG VOR.
1-) why the speed prompt was gone?
2-) why the FMA never shows the VNAV SPEED, with the speed intervention button selected and the IAS window open?
Thanks for all the info Guys... BR.
Alex.
I have a couple of doubts and I wanted know if you can give me a hand.
Well, let's begin... The plane: 734, FMC:10.7, destination: SKBO.
we were instructed to proceed BUV, VULAM, BOG. We discussed about "what if" they lead us off the lateral path with Radar Vectors, how will we do the descent? VNAV Speed, FL Change or V/S... Well, we choose VNAV Speed. When we received the first radar vector, I couldn't select the VNAV speed prompt, the option wasn't there. The other option was pressing the Speed Intervention... So, I did, but the plane remains in VNAV path, even changing the Speed. The radar vector was 15 miles before VULAM aprox, so I Was flying with a heading betwen VULAM And BOG VOR.
1-) why the speed prompt was gone?
2-) why the FMA never shows the VNAV SPEED, with the speed intervention button selected and the IAS window open?
Thanks for all the info Guys... BR.
Alex.
Join Date: Jan 2000
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1) VNAV has transitioned into ON-APPROACH mode, thus remains in PATH to facilitate CDA NPA, GNSS Baro VNAV, etc... Extend your flaps to control speed bug, or use speed intervention to manually adjust speed window whilst remaining in PATH.
2)Once VNAV is ON-APPROACH, SPEED INT allows manual control of speed while remaining in PATH.
Make us of the automatics, use VNAV PTH and Headings as you would LNAV: VNAV PTH will fly abeam the magenta line the altitude it would've had if on the magenta line, therefore increasing and reducing decent rate to facilitate CDA arrivals, great tool!
2)Once VNAV is ON-APPROACH, SPEED INT allows manual control of speed while remaining in PATH.
Make us of the automatics, use VNAV PTH and Headings as you would LNAV: VNAV PTH will fly abeam the magenta line the altitude it would've had if on the magenta line, therefore increasing and reducing decent rate to facilitate CDA arrivals, great tool!
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Why would you want to remain in VNAV PTH when not on the route? The PTH is relevant to the route in the FMC. If longer or shorter it is no longer accurate. If you select DCT TO the first relevant WPT on the expected approach after radar that will give you sensible information, but the radar vectors may take you round the houses to get there. VNAV PTH is a calculated energy profile. The best computer should be between your ears and used very frequently. KISS and use LVL CHG + any DME.
However, back to Alex77. Why were the expected prompts not available? Afraid I can't answer that one. It's a computer and does odd things, just like your laptop. The important thing is not to spend too long wondering why and trying to fix it via the FMC, but go back to basics and control the a/c via basic AFDS modes. VNAV SPD = LVL CHG (at the same speed) any way.
However, back to Alex77. Why were the expected prompts not available? Afraid I can't answer that one. It's a computer and does odd things, just like your laptop. The important thing is not to spend too long wondering why and trying to fix it via the FMC, but go back to basics and control the a/c via basic AFDS modes. VNAV SPD = LVL CHG (at the same speed) any way.
Last edited by RAT 5; 30th Jul 2015 at 14:43.
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RAT5, I do not agree with you on this I am afraid.
When CDA is mandatory at destination, the use of VNAV PTH can be very useful when being vectored around, provided the FMC reflects accurately what you are planning to do, update as you go around the lateral profile to keep you flying a CDA.
Some 737 are fitted with FMC logic enabling this, geometric descent path approach, so use it properly. Maybe you have not been in a position to fly one of them yet, as I have been with operators that have not in past or in deed current operator prior to adjusting the setup cross-fleet, it is worth it for sure.
As you stated: "Why were the expected prompts not available? Afraid I can't answer that one". I tried to answer this for the benefit of the OP:
FCOM 2, Flight Management, Navigation - Flight Management System Operation:
"In approach phase during speed intervention, the pitch mode remains in VNAV PTH after speed intervention is exited. The FMC shall remain in the current vertical mode regardless of IAS MACH selector changes."
"The following situations are generally encountered during approach operations, but are not determined by “on approach” logic:
If speed intervention is engaged:
• during a path descent with flaps up on an idle or non-idle leg, VNAV switches to VNAV SPD
• with flaps down, VNAV remains in VNAV PTH
• "on approach" with the MCP Altitude set greater than 250 feet above the current altitude, VNAV remains in VNAV PTH
• when a point to point (geometric path) leg is active, VNAV remains in VNAV PTH
• while a vertical angle leg (GPx.xx on RTE LEGS page) is active, VNAV remains in VNAV PTH
• if a vertical angle leg (GPx.xx on RTE LEGS page) becomes active, VNAV switches to VNAV PTH without pilot action
• if on a vertical angle leg, and cross track exceeds two times the RNP value, while LNAV is not engaged, VNAV will disengage."
Plenty of reasons in which VANV remains in VANV PTH, any of those could have happened to the OP...
When CDA is mandatory at destination, the use of VNAV PTH can be very useful when being vectored around, provided the FMC reflects accurately what you are planning to do, update as you go around the lateral profile to keep you flying a CDA.
Some 737 are fitted with FMC logic enabling this, geometric descent path approach, so use it properly. Maybe you have not been in a position to fly one of them yet, as I have been with operators that have not in past or in deed current operator prior to adjusting the setup cross-fleet, it is worth it for sure.
As you stated: "Why were the expected prompts not available? Afraid I can't answer that one". I tried to answer this for the benefit of the OP:
FCOM 2, Flight Management, Navigation - Flight Management System Operation:
"In approach phase during speed intervention, the pitch mode remains in VNAV PTH after speed intervention is exited. The FMC shall remain in the current vertical mode regardless of IAS MACH selector changes."
"The following situations are generally encountered during approach operations, but are not determined by “on approach” logic:
If speed intervention is engaged:
• during a path descent with flaps up on an idle or non-idle leg, VNAV switches to VNAV SPD
• with flaps down, VNAV remains in VNAV PTH
• "on approach" with the MCP Altitude set greater than 250 feet above the current altitude, VNAV remains in VNAV PTH
• when a point to point (geometric path) leg is active, VNAV remains in VNAV PTH
• while a vertical angle leg (GPx.xx on RTE LEGS page) is active, VNAV remains in VNAV PTH
• if a vertical angle leg (GPx.xx on RTE LEGS page) becomes active, VNAV switches to VNAV PTH without pilot action
• if on a vertical angle leg, and cross track exceeds two times the RNP value, while LNAV is not engaged, VNAV will disengage."
Plenty of reasons in which VANV remains in VANV PTH, any of those could have happened to the OP...
