AS332L2 Ditching off Shetland: 23rd August 2013
Rotorspeed - yes, it was a 4 axis heli being flown in 3 axis, presumably the choice of the pilots.
Perhaps worth expanding on the differences between the L2 and the 225 that could help understand that choice.
The L2 has a torque limit of 82%(?) above Vy. There is no transient allowed, so any excursion above the limit above Vy generates a HUMS overtorque warning and maintenance action. The autopilot is a digital representation of an analogue system and as such suffers from overshoots especially when stimulated by turbulence. The autopilot is unaware of the torque limit and is quite happy to raise the collective above the torque limit. Therefore one has to be very careful of power limits when using 4 axis.
This means that during high power regimes -climb, cruise - 4 axis is inappropriate. During descent the tendency is also to use 3 axis with fixed collective and vertical speed on the cyclic. The L2 likes to cruise fairly high, and likes a long descent at say 300'/min. Long periods at high speed descent in 4 axis tends to result in a fair amount of collective hunting since the AP is foolishly trying to control airspeed with cyclic and vertical mode with collective, a technique that works less well the faster you go.
However, once you get to final approach speeds (say 120 or less) 4- axis becomes appropriate as we normally want to maintain a fixed speed for an instrument approach. But you can see from the above that use of 4 axis on the L2 is used very much less than 3 axis.
By contrast on the 225 its a digital autopilot with sensor data hybridised with inertial data thus resulting in no overshoots, and minimal control hunting. It fully aware of the current power limit and will maintain a margin from the limit when coupled 4 axis. Additionally the limits have generous transient limits that can be used without maintenance penalty.
The autopilot handles high speed 4 axis cruise very well since it knows not to try to control height with collective (unlike some pilots!) and in fact the whole flight regime from hover to hover can be handled 4 axis, although the reality is that its best to use 4 axis from committal point on takeoff until 80' / 30 kts or so on an ILS.
So you can see that "normal" for an L2 is 3 axis, whereas "normal" for a 225 is 4 axis.
Perhaps worth expanding on the differences between the L2 and the 225 that could help understand that choice.
The L2 has a torque limit of 82%(?) above Vy. There is no transient allowed, so any excursion above the limit above Vy generates a HUMS overtorque warning and maintenance action. The autopilot is a digital representation of an analogue system and as such suffers from overshoots especially when stimulated by turbulence. The autopilot is unaware of the torque limit and is quite happy to raise the collective above the torque limit. Therefore one has to be very careful of power limits when using 4 axis.
This means that during high power regimes -climb, cruise - 4 axis is inappropriate. During descent the tendency is also to use 3 axis with fixed collective and vertical speed on the cyclic. The L2 likes to cruise fairly high, and likes a long descent at say 300'/min. Long periods at high speed descent in 4 axis tends to result in a fair amount of collective hunting since the AP is foolishly trying to control airspeed with cyclic and vertical mode with collective, a technique that works less well the faster you go.
However, once you get to final approach speeds (say 120 or less) 4- axis becomes appropriate as we normally want to maintain a fixed speed for an instrument approach. But you can see from the above that use of 4 axis on the L2 is used very much less than 3 axis.
By contrast on the 225 its a digital autopilot with sensor data hybridised with inertial data thus resulting in no overshoots, and minimal control hunting. It fully aware of the current power limit and will maintain a margin from the limit when coupled 4 axis. Additionally the limits have generous transient limits that can be used without maintenance penalty.
The autopilot handles high speed 4 axis cruise very well since it knows not to try to control height with collective (unlike some pilots!) and in fact the whole flight regime from hover to hover can be handled 4 axis, although the reality is that its best to use 4 axis from committal point on takeoff until 80' / 30 kts or so on an ILS.
So you can see that "normal" for an L2 is 3 axis, whereas "normal" for a 225 is 4 axis.
Last edited by HeliComparator; 23rd Oct 2013 at 08:34.
The S92 shouts "Decouple" whenever a mode is deselected - intentionally or otherwise.
I will quiz Mr. Lappos about this tonight.
Do set the Cat amongst the Pidgeons will you
Yes there is the possibility of a double-press by accident but again baseline SOPs should require mode engagement to be checked at the time of engagement.
What is difficult to cover by SOPs is the unexpected decouple without pilot action, and IMO that is the only time a "decouple" warning should be heard.
HC:
I think you have given a pretty good summary of autopilot differences however I would make the following corrections:
The L2 autopilot is aware of the TQ limit (or at least of the First Limit Indicator limit) and will NOT pull above the maximum continuous torque limit. There is anecdotal evidence of transient overshoots in turbulent conditions (which is maybe what you are alluding to) however I have not actually spoken to anyone who has had a torque exceedance due to this (maybe someone will correct me).
However what you say with regards to using the autopilot at high power settings still remains true. It’s uncomfortable in this configuration, leads to an unstable aircraft and should be avoided.
Having said all that the 4 axis autopilot does actually perform very well on IFR approaches and especially if used in conjunction with an altitude acquire (and the availability of a go-around mode) makes for a stable, controlled and safe approach.
My main point here is that your last statement is certainly not true when it comes to IFR approaches and as far as I am concerned (and the company I fly for):
A 4-axis approach (in conjunction with an altitude acquire for non-precision approaches) is very much the norm on the L2!
I think you have given a pretty good summary of autopilot differences however I would make the following corrections:
The L2 autopilot is aware of the TQ limit (or at least of the First Limit Indicator limit) and will NOT pull above the maximum continuous torque limit. There is anecdotal evidence of transient overshoots in turbulent conditions (which is maybe what you are alluding to) however I have not actually spoken to anyone who has had a torque exceedance due to this (maybe someone will correct me).
However what you say with regards to using the autopilot at high power settings still remains true. It’s uncomfortable in this configuration, leads to an unstable aircraft and should be avoided.
Having said all that the 4 axis autopilot does actually perform very well on IFR approaches and especially if used in conjunction with an altitude acquire (and the availability of a go-around mode) makes for a stable, controlled and safe approach.
My main point here is that your last statement is certainly not true when it comes to IFR approaches and as far as I am concerned (and the company I fly for):
A 4-axis approach (in conjunction with an altitude acquire for non-precision approaches) is very much the norm on the L2!
Woolf, thanks for the corrections, its 7 years since I flew an L2! I recall that the L2 didn't like being 4 axis in the cruise or climb near a power limit and had it in mind that it didn't cope with this. So how does it cope with climbing with power demands (IAS and vertical speed say) being set to higher values than achievable within the power limit? Or cruising at Vh in 4 axis (I know this is not routinely done in oil and gas, but it was done in SAR (3-axis) for a genuine emergency callouts)
As to my last para, it was my intention to show that the L2 is "normally" flown in 3 axis. By "normally" I mean for the vast majority of the overall flight time. I fully agree that everyone normally flies an instrument approach in an L2 coupled 4 axis, but the point is that, in the overall context of the flight regime, this is a pretty short period and is therefore "not normal" overall, even though it is "normal" for an IFR approach. If you get my drift!
Whereas for a 225 its "normal" to be 4 axis virtually all the time.
As to my last para, it was my intention to show that the L2 is "normally" flown in 3 axis. By "normally" I mean for the vast majority of the overall flight time. I fully agree that everyone normally flies an instrument approach in an L2 coupled 4 axis, but the point is that, in the overall context of the flight regime, this is a pretty short period and is therefore "not normal" overall, even though it is "normal" for an IFR approach. If you get my drift!
Whereas for a 225 its "normal" to be 4 axis virtually all the time.
Last edited by HeliComparator; 23rd Oct 2013 at 11:41.
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The L2 with airspeed engaged is fine if treated gently when making changes to either altitude or speed (ARAs require a little more care than the 225). It doesn't like high speeds though and it will quite happily pull above max continuous if airspeed is beeped up to higher speeds. This is often catches out ex 225 drivers when ATC ask for an acceleration from say a holding speed of 110 with IAS engaged up to say 140 - it will generally then pull above max continuous to attain the speed if, only briefly, which is why it isn't used for faster precision approaches. We fly all our NPAs four axis though using the method described above i.e. with an Alt Aquire and adjust VS for the profile.
Some warning that speed has reduced below Vy, somehow only activated when IFR (imagine slinging with a warning like that?)?
Not sure how they'd implement it,
Not sure how they'd implement it,
As regards decoupling alerts, the Sea King 3A has annunciators just under the cockpit coaming (at eye-line) that flash either P, R or C when the cyclic or collective channels are decoupled (either intentionally or through failure). The same PRC legends are on steady when the mode is engaged.
Seems to work very well without the need for additional audio warnings.
Seems to work very well without the need for additional audio warnings.
Originally Posted by [email protected]
As regards decoupling alerts, the Sea King 3A has annunciators just under the cockpit coaming (at eye-line) that flash either P, R or C when the cyclic or collective channels are decoupled (either intentionally or through failure). The same PRC legends are on steady when the mode is engaged.
Seems to work very well without the need for additional audio warnings.
Seems to work very well without the need for additional audio warnings.
Whilst you are hunting for the next group of brown people to gun down, your state of arousal is probably fairly high and you are paying attention.
Whilst we are droning along in an endless straight line over the ocean eating and reading the paper, our state of arousal is pretty low and how can we be expected to notice an ickle lightly somewhere on a screen?
The usual appeal to "play the ball not the man" is made herewith. *Return to lurking and learning about auto-pilot modes in helicopters I don't fly.*
Well I suppose I have to explain for those too dull witted to get it.
My point was that a visual indication of a critical situation with the autopilot requiring immediate pilot intervention may not be sufficient, and an audio attention getter is required.
Crab refuted this by saying he found a visual indication just fine.
I pointed out in a somewhat tongue in cheek way that there is a fundamental difference between military and civil flying which makes his opinion not relevant, namely that military flying tends to be of relatively short duration (at least in terms of annual hours) but requiring a high degree of attention. Whereas routine oil and gas civil flying tends to be long, boring and routine requiring little attention for most of the time.
In the latter case its foolish to presume that the pilots will spend an 8 hour flight (or series of flights), day after day, looking at the autopilot status indications, the more so on a type which hardly ever goes wrong. Hence the need for an aural attention getter, such as exists for the "red" master caution on the 225.
Do you get it now? Although I thought my first attempt at making the point was the more entertaining!
My point was that a visual indication of a critical situation with the autopilot requiring immediate pilot intervention may not be sufficient, and an audio attention getter is required.
Crab refuted this by saying he found a visual indication just fine.
I pointed out in a somewhat tongue in cheek way that there is a fundamental difference between military and civil flying which makes his opinion not relevant, namely that military flying tends to be of relatively short duration (at least in terms of annual hours) but requiring a high degree of attention. Whereas routine oil and gas civil flying tends to be long, boring and routine requiring little attention for most of the time.
In the latter case its foolish to presume that the pilots will spend an 8 hour flight (or series of flights), day after day, looking at the autopilot status indications, the more so on a type which hardly ever goes wrong. Hence the need for an aural attention getter, such as exists for the "red" master caution on the 225.
Do you get it now? Although I thought my first attempt at making the point was the more entertaining!
Incidently, if anyone's in the centre of Aberdeen tomorrow afternoon with a little time to spare;
The UK Oil and Gas Industry will hold it's annual service of remembrance in the Kirk of St Nicholas, Union St, Aberdeen at 2.00pm on the 26th of October. This service allows the industry to gather to remember those who have died offshore during the previous 12 months and also through the years. A candle is lit for each of those who have died in the last year and a minute’s silence is observed allowing family, friends and colleagues to remember.
Last edited by diginagain; 25th Oct 2013 at 16:39.