I was flying from Saigon to Singapore on a Singapore Airlines Boeing 787-10 when the autopilot malfunctioned. I set the VNAV to descend to 4000 feet at a rate of -500 fpm and the speed at 200 knots, but instead, the plane was descending past 4000 feet at a rate of -3000 fpm at a speed of 240 knots. The plane hit 2030 feet at the lowest point of the dive. I checked the autopilot and everything was still set the way I left it, so I don’t know why the autopilot let the plane descend and speed beyond what I let it do.
And also, I had the speedbrakes on during the descent, so that should’ve kept the speed at 200 knots.
I did experienced this when flying Dreamliner 10 around the world.
What is happening is, what you have selected are AP (autopilot), when you activate the VNAV, it calculates the altitude and the distance of the selected waypoint, and make necessary change on descend rate by overwriting the AP!
Which can happen in any aircrafts with AP and VNAV features. 🙂
The first thing I notice is you say you set to descend to 4000 feet but your altitude bug (the blue symbol on your altitude strip) shows that the altitude setting is below the current range of the altitude strip. So your altitude is actually set to something below 3000 feet.
Did your prior 4000 foot manual setting become overridden by a lower waypoint when you engaged VNAV?
Did you have VNAV pre set or did you activate it as a last second action as you were getting closer? How vnav works is you set it prior to you cruising my adding your destination elevation set to your airport. When about 2 hours out or 30 minutes from decent then reajust your flight plan and check ATIS frequentcy if on Expert server while ATC is active. Or if your not on Expert Server or there is no active ATC then hop on another device and check flightradar24 and filter your destination to see traffic patterns and runway usages. After that delete the airport out of your flight plan and set up an approach. If the airport you are landing at has STARs clear out any unnecessary altitudes that are in that STAR like anything that is 10,000 and above and any other unnecessary groups of altitudes. After that set up your finnal approach and have the initial altitude from the ILS cones set the way point to 3000ft above the airport’s elevation. Then set the runway number at the airport elevation. Once your flight plan is adjusted select the airport tap on runways and set ILS to planned runway by selecting NAV1 (note: some airport runways may not have ILS available). After that monitor your VNAV. Once your VNAV says 5nm if you are cruising at 40,000ft lower your speed to m0.80. Then once decended through 35,000ft then lower your speed to m0.70. Once below 30,000ft lower your speed to m.66. When at speed changes from M-speed to IAS reset it to 250kts unless instructed differently by IFATC. Once thats all set then wait to you hit 10,000ft and rejust your speed to 200kts (unless instructed by IFATC). Then start applying flaps as needed for 200kts. Then when on left base lower speed to 185kts and extend flaps with armed spoilers (unless instructed by IFATC). Then when turing for final lower your speed to 170kts and extend flaps again (unless instructed by IFATC). Then when at the waypoint with 3000ft lower speed to 160kts with extended flaps (unless instructed by IFATC). Then when halfway through finnal adjust speed to 150kts and extend flaps 1 more time and put the gear down (unless instructed by IFATC). Then when you hear “1000” repidly recalibrate until you feel conforable and stablized. Also disable the heading and VNAV. Then when you hear “500” disable altitude. When you hear “50, 40, 30, 20” cut throttle and get ready to flare for touchdown. Hopefully this is not to confusing and will help you become a butter pilot. If it is to confusing or you have any questions feel free to ask and i will try to help get what ever it is you are confused about sorted out.
Yes, I always have my descent profile ready before departure, check my VNAV around 100 nm before TOD, and engage the VNAV around 50 nm before TOD (I know you’re supposed to do it later, but I don’t care what you have to say about my flying lol).
That took 20 minutes for me to read and that is exactly what I do for an arrival and descent.
I don’t understand what you mean. Is -3000m fpm descent not expected from VNAV when your descent gradient is toward the steep side?
VNAV responds to waypoints with attached altitudes. For VNAV you can only choose altitude (of the waypoint) as a direct input. You can separately choose speed on the auto throttle, but that is the minimun speed. If VNAV is descending you steeply, your speed is expected to freely move higher than the autothrottle setting requiring you to manage more drag if you wish to keep the speed lower. As for fpm descent rate, you don’t choose that directly, but autothrottle set to a higher speed requires a higher descent rate, and conversely a slower descent rate for a lower forward speed setting.
On the other hand I was just playing around with VNAV and I’ve never fully explored using it to temporarily level off at an earlier altitude than the next waypoint altitude (say a stop at 10,000ft to slow down or an ATC assigned altitude). I only just now fully examined such possibilities. And then you can re-dial back in the lower altitude to resume the descent (I assume this is widely known and used). Though of course VNAV automatically calculates and sets the fpm descent rate.