RobertL18C wrote:
Perhaps the Vmc is a reduction due to increased MAUW
I can’t see how that could be, as, as you say, Vmc is at its worst when light.
In some ways, I don’t look behind why the Vmc is reduced. I rely on
I also very rarely take-off at very light weights, so I know that I have a knot or two in hand.
And, if anyone could be @rsed to work it out, there is clearly a graph of weight benefit to be drawn; that is to say, the greater acceleration associated with being lighter graphed against the Vmc at that weight. I am full of energy and enthusiasm, but fall short of doing that one!
The red line on the IAS denoting Vmc is at MAUW, windmilling propeller, aft CG, take-off configuration (but gear retracted) and up to 5 degrees of bank towards live engine. Have not seen a Vmc at an undefined ‘light weight’ before.
I think the approval is for the STC, does not imply the CAA have carried out performance tests or the new tables are Approved Flight Manual tables. If the aircraft is on an AOC presumably the AFM would have been updated for the STC.
The red line is informative – I hope no-one would use it as a target speed. The Vmc is a variable feast, so what I care about is the actual Vmc on the day. I know that that is probably slower than the worst case scenario published in the STC, so that is all I really care about.
I agree that any benefit of the VGs is for private operations only. I am now of an age that that is all that concerns me.
It does occur to me that, because both aircraft I operate are glass, I could set the red line to a calculated value before each take-off, gawd help us all.
How realistic is it to pull the power on both engines, pretend to be a single and land straight ahead into the next field? You would not be worse off than flying a single.
Or is the time required to react too short?
I wouldn’t dare trying that for myself, given the history of Vmc training accidents, and the lack of realism of the typical engine failure simulation.
Cobalt wrote:
I wouldn’t dare trying that for myself
I don’t even think the testing regime, even covers demonstration of VMC at a real take off for this class of aircraft. It’s my understanding there is an acceptable means of compliance to work it out at altitude. In any case its not an absolute, though you are definitely at the limits of control.
Cobalt wrote:
How realistic is it to pull the power on both engines, pretend to be a single and land straight ahead into the next field? You would not be worse off than flying a single.
Funny, the time I shut down an engine I was going through what I would do when the other failed. I would have been happy to land in the same way as any single and would do just that if I hadnt got a positive rate of climb for any reason, although I might hope that the power available might help to adjust the profile, rather than relying on gravity alone to do it for me.
Actually in most twins it is surprsing just how much time you have. Another myth that does the rounds is that you require split second reactions. That is not my experience. The sequence of securing an engine is hopefully smooth and rehearsed with time to establish the aircraft has a postive rate of climb, trim, and settle. There is time to establish the aircraft is still not climbing away and deal with the problem.
…. so you say that there is sufficient time to fly to the scene of the crash …
(sorry, couldn’t resist…)
That is a good point – no need to find the right field immediately, as there is in a single.
But if you are flying at Vx, which in a powerful twin will be not much above stall speed, and below Vmc? In principle, maintaining directional control is not possible with the live engine at full power, so simply dropping the nose and accelerating to above the red line is not possible, it will take too long.
So is there time to simply pull the power on the good engine, drop the nose and turn into a glider (you are no worse off than in the equivalent single), or is the time available too short and you will simply roll over?
I thought posting this ASDR for a Seneca iii might be useful. It’s from the POH so does not have public transport safety factors applied. These would apply to wind and add a minimum of 15% to the TORR. Am not sure what factor they use for the rejected deceleration run. In the absence of VGs (see @Timothy’s post), a dry tarmac runway with a TORA of 1200 metres might apply. Operating off grass would increase this materially.
Cobalt wrote:
So is there time to simply pull the power on the good engine, drop the nose and turn into a glider
This is not black and white. You can always reduce power, which will make control easier. If your airfield is at the top of a hill, like Biggin, you might then get back to a controllable speed, feed more power in and climb away.
But in extremis cutting both engines and landing in a field is always an option if things aren’t working out.
