Hello Obidiah:
I work for a Canadian manufacturer of a certain "STOL Twin Turboprop" and have just spent the last two months re-working performance data for this aircraft to enable us to publish a supplement that provides sufficient information for operators to demonstrate EU OPS-1 compliance. I kind of suspect that it's the same aircraft you are talking about (especially after having looked up that URL to the type certificate)
, but since this is an informal discussion board, and I'm not posting this response as a company employee, I'll leave it for you to deduce what aircraft I'm talking about.
The key issue here, as HarleyD pointed out in his post of July 13th and John Tullamarine pointed out in his post of July 13, is "what is the basis of certification"? In the case of (for example) a Twin Otter, the original cert basis was CAR Part 3 dated May 15, 1956, including amendments 3-1 through 3-8 plus special conditions for multi-engine turbine aircraft dated November 6, 1964.
This particular certification criteria did not require any documentation of V1, Vr, V2, etc., nor did it require that Vmc be considered when 'lift-off' speeds and speeds at 50 feet were published. For that reason, in the main body of the AFM for this aircraft, you will find the performance data is based on liftoff and 50 foot speeds that are directly related to stall speed, nothing more.
Later, the manufacturer of this aircraft elected to publish an approved AFM supplement (Supplement 11, in the case of the Twin Otter Series 300) that enabled operators who wished to do so to comply with the more restrictive requirements of SFAR 23 at amendment 1 status, published December 24 1969. This certification criteria mandated that Vmc be considered, and that V1 and V2 speeds be published. Thus, if you look in this supplement of the AFM, you will find additional, new information that is not in the basic body of the AFM. This new information includes the following:
- Vmc (air only, no Vmc ground was ever published)
- V1 and V2 speeds (but note, no Vr speeds were ever published)
- accelerate-slow distance (to a 35 knot speed)
- landing distances based on a different speed criteria (1.3 Vs at 50 feet, rather than 1.5 Vs at 50 feet).
In other publications, such as a unique AFM published in the 1970s to comply with the regulatory requirements of the UK (BCARs, as they were known then), you will find performance graphs that present a Vmu and some factored wind data. There was also a unique AFM published in the 1970s to speak to Australian DCA regulations affecting the mainland and what was then known as the Territories of Papua and New Guinea, but compliance with contemporary CASA regulations has since been subsumed into the main AFM by way of a supplement first issued in 2000.
The problem, though, is that you cannot 'mix and match' data from these three different certification standards, otherwise, you will wind up with a massive headache such as you described in your first post. You have to pick one cert basis that you want to work with, then confine your data analysis and preflight calculation to that cert basis only. For most operators who want to use conservative data, that means the SFAR 23 supplement.
Now, having said all that, let me try and address your question about raising the nosewheel. First of all, there is no Vmcg for this aircraft, it has never been published, and it is likely that it will never be published. The only benefit of determining and publishing a Vmcg (from a performance analysis perspective) is that it allows a lower V1 to be published if that would be advantageous for balanced field length calculations. In the case of the Twin Otter, if you do performance calculations on any cert basis from SFAR 23 onward (e.g. FAR 23 or even FAR 25), the overriding limiting factor for V1 is the requirement that it be 1.1 times greater than Vmc. This yields 73 knots, which is a pretty high number, hence there is no benefit of any kind arising from determining Vmcg for BFL purposes. Besides, accelerate-go is always the longer distance for a Twin Otter, not accelerate-stop.
You mentioned '79 knots' in your original post. I don't know where that number is coming from. Using the SFAR 23 data (supplement 11), at gross weight, V1 is 75 and V2 is 80. Vr has never been published (such is the nature of SFAR 23 at amendment 1), but it can be safely assumed that most pilots will initiate rotation as soon as V1 is reached, to enable them to reach (without overshooting) V2, which is 80.
The technique that was used during the SFAR 23 cert testing of this particular aircraft was as follows: The pilots would accelerate to V1, allowing the elevator to 'come alive' on its own - neither holding it forward or aft - then they would rotate to achieve V2 as soon as V1 was reached. I'm not saying you should or should not do that, I'm just saying that was the technique used (according to what was set out in the cert basis), and that technique will give you 'book results', at least, at any weight above 12,000 lbs. Below 12,000 lbs, things get tricky. V1 below 12,000 lbs is limited to 73 knots at any weight by the requirement to stay 110% above Vmc, and at very light weights, this particular aircraft will become airborne on its own prior to V1 - this being a consequence of the very high lift wing design. I guess that in those circumstances, you might need to press forward a bit to avoid premature rotation if you wanted to strictly respect the V1 speed.
You asked what the "true and proper" definition of Vr was. Well, in the case of the cert basis used for this aircraft (CAR 3 and SFAR 23), there is no Vr. The regulations didn't mandate that it be published, thus, it was never published. I can appreciate your uncertainty - you are looking for something that doesn't exist.
Dan Winterland made reference to a Vmu in his post. It is true that Vmu figures were published in the BCAR version of the Twin Otter AFM, but this particular book was written to comply with BCAR mandated technique, and you can't transfer the Vmu from that publication over to the Vr and V1 graphs in the SFAR 23 publication - the rules are not the same. It would be like trying to add inches and centimeters together to get a total.
As for STOL - no regulatory authority ever published a cert basis for STOL operation of a civil registered aircraft. The BCARs came closest - they did address a 'short field' technique, but this wasn't really STOL. If you look very carefully at the STOL section of the Twin Otter AFM, you will see that there is a preface on the first page that says that all the performance data in that section is totally unapproved, and prior permission is required from your local regulatory authority before that technique (and any of that data) may be used.
Finally, Brian Abraham asked a question in his post of July 14th about the restriction on landing weight (no more than 11,000 lbs in temperatures of -20). The reason for this is because the shock absorption during landing on a Twin Otter is accomplished by urethane blocks and by the rubber main wheels. Below -20, both of these items become less resilient and cannot dampen shock satisfactorily at the maximum rate of descent specified in the cert basis if the landing weight is greater than 11,000 lbs. FYI, there is also a requirement to lower the air pressure in the main wheels when landing in temperatures below -20... again, this is because the main wheels are part of the shock absorption system. It has nothing to do with engine performance or missed approach performance, it is entirely a shock absorption issue. You were correct when you suggested this in your post of July 15th.
Hope this information assists you. Like I said, it is informal, I'm not talking on behalf of my employer. Hopefully a supplement containing far more information, to enable operators to comply with EU OPS-1, will be published in the near future. This supplement will contain the 'missing data' that was not required by SFAR 23 at amendment 1. I have to admit, though, that it is kind of amusing to calculate 4 state takeoff climb requirements for a fixed gear aircraft that uses the same speed and flap setting for V2, initial climb with 2 engines, or initial climb with 1 engine out. Uh, what third stage? Just rotate and fly at 80 knots and flaps 10, no matter what happens, until you get to 1,500 feet AGL.
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