Problems with climb-out on B733

[cdp]

 I do have a real problem controlling the plane on climb-out. The initial climb-out is fine, speed is controlled on MCP, LNAV engaged and VS set to initially 2000ft, which I then slowly reduce as we get higher and the speed then reduces on the ASI. I do trim the aircraft and slowly increase speed on MCP until about 270knots. Things are fine until about FL160, VS now about 1000ft, when airspeed suddenly decreases until a stall occurs. No input from my side can change anything. This has now happened numerous times. I've gone thru all the Tutorials, but fail to find anything. What am I doing wrong? Any help out there? I use WIN10 with the latest update, have an I7 processor and a Nvidea GTX1080 graphics card. Performance is very smooth, but the behavior of the plane not right.
 

Edited October 12, 2019 by cdp
Unclear what plane

[SkyFly]

Don´t use VS.
Use LVL CHG instead.

Also: Make sure you are not overweight.

Edited October 12, 2019 by SkyFly

[Litjan]

If you want to, take some screenshots around the incident, just before, during, just after. Make sure we can see the modes engaged on the autopilot, then engine instruments, the primary flight instruments.

Normally the autothrust will control the engines during a climb, with nominal power being climb thrust. This is signified by the "N1" showing on the mode annunciator in the electronic attitude indicator. The pitch of the aircraft is normally controlled by the autopilot to maintain a set airspeed (FL CHG mode) and should show "MCP SPD".

It sounds like you are climbing in vertical speed mode with the thrust controlling the airspeed, which is only recommended for short (step) climbs. However there are still some protection modes active that should kick in to save the aircraft in case of gross pilot error, so I am not sure how you get it to stall...

Another possibility is windshear or icing, try to fly with the weather set to no clouds and no winds and see if that changes anything.

Cheers, Jan

 

[Iain]

For icing, have a look outside at the wiper blade, if ice is building up on it, switch on the anti ice on the overhead panel.

 

[cdpohl1]

Thanks to all who responded. I guess I must be real stupid, but I just had the same problem again on a short hop from ELLX to EBBR, when the plane, on passing FL160, rapidly reduces speed and subsequently stalls. I used the Tutorial 'Full Flight with FMS' as guidance. Used SimBrief for my briefing. Here a short description: ZFW 46.3, Gas 4.9 with 2 as reserve, cruise level FL200. Settings on takeoff: Both FDs on, A/T armed, N1 engaged for takeoff. LNAV engaged after airborne. Unable to select VNAV. No problem thus far, aircraft climbing. N1 changes to SPEED. Increasing speed on MCP to 270 passing FL100. Aircraft is slowly increasing from 230 but does not reach the set speed of 270. Rate of climb reduces steadily, then speed reduces quickly and a stall occurs. No icing present.

I'm kinda annoyed now since it has happened so many times. I will try and record the settings on my next doomed adventure. Meanwhile thanks again.

[Litjan]

Looking forward to your pics, I am sure we can figure out whats going wrong for you there.

Cheers, Jan

 

[cdpohl1]

Right, here we are again. Repeated again the short flight ELLX to EBBR, and, believe it or not, encountered the same problem again. N1, VNAV and LNAV engaged and climb-out fine until I reach FL200 when all hell breaks loose again. Soon after I find myself in pieces back on the ground, still breathing, though. I have attached a zip file with pictures, and hope someone can shed light on what the heck I'm doing wrong here. Thanks in advance.

 

Photos(1).zip

Edited October 13, 2019 by cdpohl1
Not opening up.

[cdpohl1]

Have difficulties attaching small jpeg files. Will keep trying.

[Litjan]

Hmm, it is hard to say because your pictures only show part of the cockpit, but it looks like you are cruising in ALT HLD mode past your calculated top of descent. In that case the plane can NOT leave the cruising altitude, but it will reduce the commanded speed to "trade energy" - it should not get too slow, though. As one can see in your second to last picture, the commanded speed is 210 kts, and the yellow FMC light is on (probably due to the RESET MCP ALT warning that you got when approaching your top of descent).

The thing that is really weird is the commanded speed of 45246 kts - this is clearly a bug and probably due to the speed reversion protection cutting in (you can see the engines going to full power), but not the reason of your problem.

Try to pay attention to where the calculated T/D point is on your routing - and start the descent accordingly. It is IMPERATIVE to dial down the MCP ALT to enable the descent, otherwise the plane will never leave the altitude.

I also recommend to NOT fly the descent in VNAV. It is not always behaving right during the descent (especially with multiple restrictions on the descent path). Use FL CHG or V/S mode to descend.

Let me know how that goes, Jan

 

[cdpohl1]

Ok, thanks for your reply. I did reach FL200 just north of DIK, still quite a ways from TD. On reaching FL200, N1 disappeared and so did VNAV. The speed then quickly reduced and the aircraft stalled.

[Litjan]

Hmm, but looking at your picture I can see that you are in the descent mode of the FMS, that is only triggered by passing the calculated T/D. Also the path indicator shows you a few thousand feet high on the descent path, so something doesn´t quite match your description.

If you are flying to Brussels and are close to Olno like you are in your picture, you are maybe 50NM from your destination and should definitely be in the descent portion of your flight.

The distance from Luxembourg to Brussels is just over 100NM, and FL200 is way too high for that distance, chances are that you can´t even reach it before you need to descend again.

Try a longer distance flight, or fly at maybe 160 or so.

Cheers, Jan

 

[cdpohl1]

Hi Jan,

Again thanks for your input. I have heeded what you recommended and succeeded finally to complete the flight. What did I do? On attaining FL180 ( Simbrief recommended FL200, which I changed it to FL180), I deselected VNAV and continued on MCP speed (reduced it from 330 to 280), and the for descent I used V/S, as suggested by you. Lo and behold, I managed to sneak into EBBR. I was very happy to finally succeed, as you can imagine. I do think, however, things are not what they should be. For example, if you continue on a path that is dangerous, there should be warning bells, or some other  indication. Why would the plane suddenly reduce its speed and stall without any prior warning? Really has me baffled. Nonetheless, I will now try other flight sectors to ascertain that the settings I used work. If they do not, I will get back to you, and I hope you don't mind. Thanks again for your help.

[Litjan]

Hi,

you are totally right, of course. The real plane wouldn´t do that - I accept the blame on our "less than perfect" VNAV implication. We have repeatedly acknowledged that our VNAV is - especially in the descent - fairly buggy, especially as soon as you go off the "beaten path" and expect it to handle unusual situations.

Nevertheless - even the real plane´s automation can fail and it actually is the reason why there are pilots in the cockpit (besides looking good ;-)) - to save the day when the automation fails.  I recommend mapping the "autopilot disconnect" buttons to a joystick button and taking control of the aircraft when something obviously goes wrong. Plummeting to earth from 20.000 feet is a good indication that something isn´t right, even if there are no warning bells (I do believe there must have been a stick-shaker, though?).

In reality we watch VNAV like a hawk when it flies the aircraft. I know that many simmers think that LNAV and VNAV are the magic buttons that they can use to fly from A to B without worry, and if everything works well, they are. In reality I saw less than 5% of all pilots use VNAV during descent, in both the 737-300/400/500s and 747-430s I flew. Just to show you how much we trust the real VNAV to do what we want ;-)

Cheers, Jan

 

[Iain]

In the case where vnav isn’t engaging, you can help prevent this during fmc programming.  Is sometimes a problem with the fmc design but in my experience is usually human error.

Keep your FL realistic, an easy way is to check flight radar 24 and look at a previous flight that flew the route and see what level they used.  I do this for short flights like Edinburgh to Dublin, bear in mind our old lady takes a little longer that a nice -800.  

 

Make sure that the route is built correctly including sids and stars, review it using the legs page and nav mode plan so you can step brought it.

Make sure your pref page is complete and that you activate it.

The most important and common reason I can sometimes not get vnav to engage is after you have done all off the above,  check the legs page again and you should now see and speed or height restrictions on the right side of the fmc legs page, if any of the waypoints is missing a restriction or an estimated height, the vnav won’t be happy and won’t engage.  This can be fixed before or after takeoff but is easier to sort out before takeoff.  Missing info can be manually entered or if a waypoint isn’t working out just delete it.

Also check that when you try to engage vnav that the aircraft is at a speed and height that it expects ie. the fmc legs says 250kts  and 6000ft for the next waypoint but you are at 8000ft, it won’t engage.

Vnav works really well for me, in descents too, i usually switch to flt change or vs below 10’000ft anyway but it can work all the way down to the ils.  

Final tip, for short flights, try a high cost index in the perf page, something like 140, it gives you a nice climb, high cruise etc.

Edited October 13, 2019 by Iain

[cdpohl1]

Thanks for all the input and help from your side. I'll try to keep all this in mind. My last flight was successful, VNAV and LNAV worked fine, although I now tend to deselect VNAV on reaching top of climb.