Asymmetric go-around decision height in light twins
Oktas8,
In my region, CASA "require" blue line speed to the mythical "commit point", allegedly somewhere on short final.
In something like a little Partenavia P-68, for example, there is quite a gap between "blue line" speed, and a sensible approach Vref, resulting in a very unstable short final.
What I see, sitting watching having lunch, all to often, is 'orrible, with an aircraft floating 1500 ft or more, trying to wash of the excessive speed.
Unsurprisingly, these very poor (euphemism bloody dangerous) "techniques" carry through, resulting in speeds "over the fence" in aircraft like Metro anything up to 20kt above Vref.
Most of us can remember a Metro, engine out, going off the end and being badly damaged at Mckay, simply because the speed over the fence was no where near an appropriate Vref under the circumstances.
I can think of a number of cases of Metro off the end, all the same cause, and plenty of cooked brakes.
Tootle pip!!
In my region, CASA "require" blue line speed to the mythical "commit point", allegedly somewhere on short final.
In something like a little Partenavia P-68, for example, there is quite a gap between "blue line" speed, and a sensible approach Vref, resulting in a very unstable short final.
What I see, sitting watching having lunch, all to often, is 'orrible, with an aircraft floating 1500 ft or more, trying to wash of the excessive speed.
Unsurprisingly, these very poor (euphemism bloody dangerous) "techniques" carry through, resulting in speeds "over the fence" in aircraft like Metro anything up to 20kt above Vref.
Most of us can remember a Metro, engine out, going off the end and being badly damaged at Mckay, simply because the speed over the fence was no where near an appropriate Vref under the circumstances.
I can think of a number of cases of Metro off the end, all the same cause, and plenty of cooked brakes.
Tootle pip!!
The controversy seems to be the insistence of some Examiners that blue line speed must be maintained all the way down final approach when OEI. Further compounded by some who want blue line even with all engines operative on the basis that an engine 'might' fail.
The three speeds which will kill you if not absolutely respected are: Vs, Vmca and Vmcl.
There comes a time on every approach in most light twins (there are a couple of low performance exceptions) when blue line speed needs to be thrown away in favour of a stabilised approach speed which could be considerably less. An extreme example of this is the MU-2 which has a split between blue line and Vref of about 60 knots.
As long as you are headed down the slope, going below blue line on one engine is unlikely to kill you. BUT, if you go below Vmcl you can not safely apply power if you get too low. Vmcl is a little understood speed. It will probably be less than blue line and may or may not be below Vref. If Vmcl is above Vref, then Vmcl would be a wise speed to respect when OEI. If engines are performing as advertised, reasonable risk management means Vref is more relevant. Vref plus 5 knots is a good target and one I normally use unless it is very gusty, when plus 10 is a nice number. Considering that Vref is already either 1.25 or 1.3 Vs, then Vref plus 15 is too fast!
If Vmcl is below Vref, lucky you - you are probably in a nice forgiving twin. Not that I am very familiar with it, but I think the BN 2 Islander is in that category, i.e. Vmca and Vmcl are less than recommended approach speeds. The problem is, Vmcl is not a speed that is published in every light aircraft POH. Also, published Vref or recommended Vapp may not have any correlation to it in this class of aircraft. In bugsmasher-land you are sometimes left to establish these things for yourself.
Whether stabilisation is three miles/900 feet (e.g. my choice for the aforementioned MU-2), two miles/600 feet (quite appropriate for most, including most turboprops) or one mile/300 feet (OK for Aztecs etc where there is not much of a split between the various speeds) really is what should be emphasised in training.
The three speeds which will kill you if not absolutely respected are: Vs, Vmca and Vmcl.
There comes a time on every approach in most light twins (there are a couple of low performance exceptions) when blue line speed needs to be thrown away in favour of a stabilised approach speed which could be considerably less. An extreme example of this is the MU-2 which has a split between blue line and Vref of about 60 knots.
As long as you are headed down the slope, going below blue line on one engine is unlikely to kill you. BUT, if you go below Vmcl you can not safely apply power if you get too low. Vmcl is a little understood speed. It will probably be less than blue line and may or may not be below Vref. If Vmcl is above Vref, then Vmcl would be a wise speed to respect when OEI. If engines are performing as advertised, reasonable risk management means Vref is more relevant. Vref plus 5 knots is a good target and one I normally use unless it is very gusty, when plus 10 is a nice number. Considering that Vref is already either 1.25 or 1.3 Vs, then Vref plus 15 is too fast!
If Vmcl is below Vref, lucky you - you are probably in a nice forgiving twin. Not that I am very familiar with it, but I think the BN 2 Islander is in that category, i.e. Vmca and Vmcl are less than recommended approach speeds. The problem is, Vmcl is not a speed that is published in every light aircraft POH. Also, published Vref or recommended Vapp may not have any correlation to it in this class of aircraft. In bugsmasher-land you are sometimes left to establish these things for yourself.
Whether stabilisation is three miles/900 feet (e.g. my choice for the aforementioned MU-2), two miles/600 feet (quite appropriate for most, including most turboprops) or one mile/300 feet (OK for Aztecs etc where there is not much of a split between the various speeds) really is what should be emphasised in training.
Last edited by Mach E Avelli; 7th Dec 2014 at 02:30.
Further compounded by some who want blue line even with all engines operative on the basis that an engine 'might' fail.
Guaranteed 100% unstable approaches for 100% of operations, to cover a 10-4 or less occurrence.
Risk management from the Campaign Against Safe Aviation.
It's years ago, now, but it took us a while, in an airline training environment, to figure out why so many new intake pilots had such problems with a stable approach --- until we realised that in their GA years, CASA had dictated unstable approaches.
Tootle pip!!
