The FAA is very keen on this, although for Part 91 it is only advisory in nature unless the AFM states it as an operating limitation. I believe EASA investigated ASDR for non commercial air transport but may have decided not to implement the rule when it realised that around 900 GA airports would be affected (not talking farm strips or glider sites here).
It is not an article of faith, so I would expect we all agree, that the most dangerous interval for a multi engine to lose an engine is between Vmc and Vyse (blue line) on the take off run. Hence the FAA emphasising that in a multi engine ASDR should be calculated.
I don’t know about the Tecnam MEP, but the Diamond DA42 does not appear to publish an ASDR performance table (it only publishes a TODR, so presumably you need a TORA of at least the TODR to 50 foot barrier height).
The practical effect of calculating ASDR is that most MEPs need 1000 metres plus of TORA, the Islander probably exempted, if they plan to comply with ASDR including the inevitable ‘startle’ effect of rejecting a take off. I would suggest the DA42 will not stop within the TODR if take off is rejected at around Vtoss (take of safety speed).
Pretty standard take off briefs for Performance B (all GA MEPs) use language along the lines of ‘if the malfunction occurs after Vyse, with insufficient runway remaining, will carry the defect into the air and deal with it according to the procedures’. This suggests that knowing whether there is sufficient runway would be useful.
Does this (no asda required) also apply if you operate e.g. a DA42 commercially (AOC ops)?
Presumably the lack of interest is that it would destroy the utility value of twins, in much of Europe.
RobertL18C wrote:
I believe EASA investigated ASDR for non commercial air transport but may have decided not to implement the rule when it realised that around 900 GA airports would be affected (not talking farm strips or glider sites here).
EASA intended for some time to apply balanced field length requirements to twin engine turboprops < 5700 kg operating under Part-NCC. It eventually put in an exception, meaning that it applies only to jets or heavier turboprops. NCC.POL.125(b).
Coincidentally I looked into the accelerate-stop distance a few days ago, and was surprised to find that for Part-23/CS-23 the TC holder has to establish ASDR only for commuter category aeroplanes. CS 23.1587(d).
Without wishing to intrude on the SEP vs MEP debate, I think I would be much less comfortable operating a twin on a regular basis from a runway that did not allow me to get to Vyse and stop again than from a runway with plenty of length (as I do). In the case of a non-catastrophic problem, the decision-making process is probably easier in a single. In a twin, a precautionary approach to an engine glitch on a short runway may mean an encounter with a hedge or worse.
From my AOC experience the CAA approved AFM, with Public Transport safety factors applied to performance tables, did not include ASDR tables. However, for a IFR AOC flight you would compile a Performance B Net Take Off Flight Path calculation. Depending on obstacles in the vicinity, the NTOFP would usually impose a certain minimum ceiling before entering IMC for the average GA piston twin. The NTOFP assumes you lose an engine as you enter IMC, while if you maintain VMC you are allowed to manoeuvre around an obstacle within certain parameters. The PT safety factor adjusted tables for SE ROC leave you with little or no margin to achieve obstacle clearance if the ceiling is low.
In practical terms AOC IFR operations in MEP require quite conservative ceilings and visibility for an IFR departure with modest obstacles in the departure area. As a working assumption I would be using 600 feet and 2 miles due to a 400’ obstacle which I needed to see on line up.
Either way, using an ASDR which would mean operating from larger airports, or using PT safety factor NTOFP, the MEP has constraints if it is to deliver consistently the safety advantage of a second engine.
There are other workarounds. I discovered that by fitting vortex generators to a PA31 (Navajo and Chieftain) I could reduce the rotate speed to the point where a 800m accelerate stop is achievable.
How does this relate to Take off safety speed which requires a margin above Vmc (10-15% plus)? The Beech Baron has tremendous short field performance with 25 degrees flap, except Vx in that configuration is below Vmc, with Vmc rolls in this configuration to prove it in the NTSB.
Did the VGs come with new performance tables for the PA31 AFM? At Vx (the calculus of Performance B ASDR I believe is keyed off Vx) there is 3000Kj to dissipate, so an ASDR of 800m implies impressive braking power for brakes without ABS.
The Twin Otter when operating off gravel I believe is allowed to lift off before Vtoss, but the pilot then immediately pushes to stay in ground effect.
It is certified and does come with performance data.
As you can see the Vmc (which is worst case scenario, light weight, aft CG and windmilling) is also reduced. It sets Vlof at 77kts, which is only Vmc + 5 (ie <10%) so there is a small element of risk there.
The original AFM actually allows a short field rotation, with one stage of flap, at the original Vmc, which seems a little, er, optimistic.
I use a slightly bastardised version of accelerate stop, with the short field figures, but I don’t deploy flaps:
You can see that at sea level (the main base is Fairoaks), even at above average temperatures and nil wind, we are sub 800m. At ISA and with some wind, there is no issue at all.
The Chieftain does have pretty good brakes. They can be further upgraded, but my experience is that the tyres give up before the brakes do (ie skidding can easily be induced by the current brakes) so there seems little point in getting greater braking power (though I would love to be told that the upgraded brakes improve that.)
Yes, it is a balance of risk. Of course, one would prefer to rotate at blue line on a long runway. @bookworm is lucky that he shares his nearest airfield with airliners coming in for maintenance, but if for operational reasons you need to use normal GA airfields, all you can do is make what modifications you can, go in with your eyes open, do the calculations, decide what risks are acceptable and adapt your technique as necessary.
…oh, and I put particular focus on reducing recognition time. My eyes are focussed on the dials, and I know exactly what I won’t accept.
However, as we know, total immediate engine failure are very rare indeed. As APS is constantly telling us, we do not fly twins, we fly duodecims. (Except Bookie, he flies an octave.) That’s twelve largely independent engines sharing two crank shafts and some common components, most of which are duplicated.
Yes, total immediate engine failures can happen, a broken crankshaft for example (though I wonder whether under those circumstances you’d get windmilling), but most engine failures take time. Even when I had a cylinder through the cowling, the engine kept going, albeit very roughly. Much more common is the loss of one cylinder due to plugs, or the loss of a magneto. Even a fire will keep turning and, er, burning as you rotate.
That means that the chances of a total engine failure from hero to zero at that split second of 77kts is really very unlikely. If I got bad indications at 77kts (say the loss of a cylinder, rough running and a drop in RPM) I would take it into the air, get the gear up, blue line and then shut down. We are only talking a few seconds.
That does appear to be a workaround. I can see how VGs reduce Vs, need to think through how Vmc was reduced. Vmc reduces with higher weight (somewhat counterintuitively), because Lift=Weight in un accelerated flight, therefore the horizontal component of lift allowed for Vmc increases, reducing Vmc. Perhaps the Vmc is a reduction due to increased MAUW, and a slightly lower pitch angle reducing P-effect? At lighter weights CG tends to move forward so there is something of a natural compensation, as rudder moment increases and Vmc is calculated at aft CG.
Technically it is a POH supplement, not a CAA Public Transport factored Approved Flight Manual.
Thank you for posting, may be the first time someone posted an ASDR table here.
