Agree, quatrelle. That “it works fine if you limit VS to +500” doesn’t hold up in practice. There are basically two cases where VS mode in climb can potentially get you into trouble. The first is where one is not vigilant of the (natural) decrease in power in an NA aircraft and forgets to manually decreasing the selected VS rate. But really, is anybody that asleep during climb that he doesn’t notice anything during the entire time the speed slowly goes from Vy plus x all the way down to stallspeedm with all the warning signs one gets before that point? The other case might be a major engine problem or a strong downdraft. But in these cases, even limiting the selected climbrate to 500 fpm (even if the aircraft could “normally” do say 800 at Vy) will not help to avoid a stall.
boscomantico wrote:
The other case might be a major engine problem or a strong downdraft. But in these cases, even limiting the selected climbrate to 500 fpm (even if the aircraft could “normally” do say 800 at Vy) will not help to avoid a stall.
In that case, an autopilot could theoretically also fly you into a stall in altitude hold mode.
Yes; I agree with the above.
That said, I normally climb in VS mode, adjusting it down as I climb.
If climbing to a low altitude, say 5000ft, one can just set +700fpm and it will hold it all the way up, with the IAS dropping to some 110kt at 5000ft.
When approaching the ceiling, say above FL180, the VS mode is no good on my autopilot (KFC225) because it can be adjusted in 100fpm steps only and furthermore the autopilot randomly knocks 100fpm off the set figure (a bug, obviously). So, given the ceiling is defined as a 100fpm rate of climb (on an N-reg; on a G it is +50fpm on the TB20 so a G-reg TB20 climbs another 2000ft 
) this isn’t going to get you there 
So for that I use the PIT (constant pitch) mode. The pitch attitude in PIT mode, like the VS in VS mode, can be adjusted using the + and – buttons. Also the pitch can be adjusted using CWS.
If one is not paying attention, the stall warner comes on I have often been at FL180-210 with the stall warner going, and sometimes literally stalling, while trying to stay above some IMC.
If in IMC and busy due to some stuff going on, one might not notice the IAS bleeding off when in VS mode. I have done it once or twice. But the same will happen in PIT mode; it just happens later, as you approach the ceiling – because the engine is getting less air, making less power, and unable to maintain the set pitch because it has run out of power to pull the drag corresponding to that pitch.
And the IAS-hold mode which you get in more recent autopilots isn’t going to work either to reach the ceiling unless you climb at a low IAS figure, say 90kt, but climbing at 90kt at low level and the high power setting is awfully bad engine management, due to poor airflow…
So every autopilot mode will fall over eventually as you climb higher.
On jets you have masses of performance and short of doing an AF447 you can climb at a constant IAS or a constant mach, to any altitude where you actually want to be for fuel efficiency.
Peter wrote:
On jets you have masses of performance …
But even on jets the performance degrades with mass, altitude and ISA+ conditions. I remember last summer I did a flight with a Citation Bravo (which I fly once in a while, usually it is an Encore) from southern France to Farnborugh. Therefore accepted a climb at max. take-off mass straight up to FL430. Luckily I have the habit to use FLC (flight level change or constant IAS) for climbs, which the Bravo is not really good at because the autopilot tends to end up in pitch oscillations chasing the airspeed, because at something like FL380 it stopped climbing altogether. Had I used VS (which for passenger comfort is the better alternative) the stick shaker would have woken us up at FL400…
Never stop to monitor your instruments! Single pilot that can be a challenge at busy times, but in a multi-crew environment there is zero excuse for not getting an early warning that something is about to get wrong very soon.
boscomantico wrote:
The first is where one is not vigilant of the (natural) decrease in power in an NA aircraft and forgets to manually decreasing the selected VS rate. But really, is anybody that asleep during climb that he doesn’t notice anything during the entire time the speed slowly goes from Vy plus x all the way down to stallspeedm with all the warning signs one gets before that point?
Of course. No autopilot is a reason to no longer scan your instrumentes. But I guess this case is the reason they put the warning in the 55X if the climb rate decreases below the selected rate too.
boscomantico wrote:
The other case might be a major engine problem or a strong downdraft
True. And in both cases the AP will fly the plane into a stall even in ALT HOLD unless action is taken.
boscomantico wrote:
But in these cases, even limiting the selected climbrate to 500 fpm (even if the aircraft could “normally” do say 800 at Vy) will not help to avoid a stall.
Exactly. Particularly in the latter case, the first course of action would be to turn the AP off, at least initially, get the airplane sorted and if there is a condition in which the AP can help, re-engage it.
Peter wrote:
That said, I normally climb in VS mode, adjusting it down as I climb.If climbing to a low altitude, say 5000ft, one can just set +700fpm and it will hold it all the way up, with the IAS dropping to some 110kt at 5000ft.
That is what we do. The M20C will usually hold significantly more than 500 fpm up to 10k ft, double that much at SL and about 600 fpm at 10k ft, above that we use 300 fpm as SOP and reduce it as required to keep the speed. Of course speed has to be monitored at all times and if it goes below 105 mph (cruise climb) then we adjust VS accordingly, well before any alert.
Peter wrote:
The pitch attitude in PIT mode, like the VS in VS mode, can be adjusted using the + and – buttons. Also the pitch can be adjusted using CWS.
This does not work in the STEC 55x as the way CWS works there is that it reads the current VS and keeps it… which is kind of cool if you do a descending 360° turn but not so much for what you do with the King one.
Peter wrote:
And the IAS-hold mode which you get in more recent autopilots isn’t going to work either to reach the ceiling unless you climb at a low IAS figure, say 90kt, but climbing at 90kt at low level and the high power setting is awfully bad engine management, due to poor airflow…
What IAS hold will do is to hold the selected IAS vs a set power setting. It will not stall the airplane (unless selected speed is below Vs) but it will fly a profile by which the set thrust defines the ROC/D. I regard it as MUCH safer than VS in any case, but of course it is not foolproof.
But that does not say that you can’t use VS if you know what you are doing AND, as always when you use an AP, are ready to take manual control at any moment.
Peter wrote:
On jets you have masses of performance and short of doing an AF447 you can climb at a constant IAS or a constant mach, to any altitude where you actually want to be for fuel efficiency.
On heavy jets you follow an IAS/MACH profile, mostly by FMS. The principle is the same, but the technical side is different in so far that the IAS/MACH will be set by the climb profile you put into the FMS. In the MD11 we had 250/290/M.80 for climb. 250kts below 10k ft, 290 until it matches .80 and .80 thereafter.
Michael wrote:
The mechanic that used to maintain N121AG at Deurne is a very good friend. The plane had just come out of maintenance and if I remember the story correctly they were over gross, IMC , and climbing out when the pilot lost control, not sure if it was an AP incident or a IA failure, but they are very lucky to have survived. There was an investigation by the authorities, but it was considered pilots’ fault.
I have found the full story (in powerpoint format printout …) in my aircraft’s documentation.
There was a crew of two, in IMC. Pilot Not Flying suddenly noticed that the Attitude Indicator was showing a 90 degree bank (but there was no matching yoke movement). He took control (?!) yanked the aircraft back into what he thought was horizontal and almost did a roll.
It seems obvious that it was a failure of the Attitude Indicator coupled with an inappropriate reaction by the PNF. If I were the PF at this point I would have probably activated his ejection seat and kicked his ass on the way out…
They eventually recovered and decided to… buy a non-certified Dynon AI which I remember seeing on pics of the panel before delivery to me.
I am relieved that the cause was not the STEC55X and that all of the potential suspects are now removed from this aircraft – including PNF!
