Use the Autofeather

[pelmet]

I thought the 2Otter had a reduced T/O weight for autofeather inop...

No, that's not the case. If I recall correctly, there is a slight increase in Vr - which results in a longer accelerate-stop distance - if the autofeather system on a Series 300 or Series 400 Twin Otter is inoperative. There is no weight reduction.

Going from memory, I believe that autofeather reduced the VMCa from 68 to 64 knots. Maybe somebody can confirm. It would be interesting to know if this is the same on the -400 series.

[PanEuropean]

Going from memory, I believe that autofeather reduced the VMCa from 68 to 64 knots. Maybe somebody can confirm. It would be interesting to know if this is the same on the -400 series.

Hi Pelmet:
Your memory is pretty good, although you have the numbers reversed. The AFM supplement that addresses operation with the autofeather system inoperative states that when the autofeather system is inoperative, the pilot should consider the Vmc to be 68 KIAS, rather than 64 KIAS.

In practice, the Vmca doesn't change (obviously, the aircraft doesn't know that the autofeather system is inoperative). The purpose of the 4 knot increase is to push V1 and Vr up to no less than 68 KIAS, this to allow for a potential loss of 4 knots airspeed while the pilot recognizes the asymmetry and manually identifies and feathers the propeller. V2 also gets pushed up 4 knots from its normal value for the same reason.

[goingnowherefast]

Wouldn't the 4 kts relate to a feathered vs unfeathered prop? A failed engine windmilling will have more drag, more yaw and have a higher Vmca. The pilot still has to control the plane while the prop is manually feathered. Autofeather takes care of that much faster.

[digits_]

Wouldn't the 4 kts relate to a feathered vs unfeathered prop? A failed engine windmilling will have more drag, more yaw and have a higher Vmca. The pilot still has to control the plane while the prop is manually feathered. Autofeather takes care of that much faster.

I don't think there are unfeathered engine failed performance numbers, but could be wrong. I'd think the Vmca difference would most likely be much more than 4 kts though.

[pelmet]

Going from memory, I believe that autofeather reduced the VMCa from 68 to 64 knots. Maybe somebody can confirm. It would be interesting to know if this is the same on the -400 series.

Hi Pelmet:
Your memory is pretty good, although you have the numbers reversed. The AFM supplement that addresses operation with the autofeather system inoperative states that when the autofeather system is inoperative, the pilot should consider the Vmc to be 68 KIAS, rather than 64 KIAS.

In practice, the Vmca doesn't change (obviously, the aircraft doesn't know that the autofeather system is inoperative). The purpose of the 4 knot increase is to push V1 and Vr up to no less than 68 KIAS, this to allow for a potential loss of 4 knots airspeed while the pilot recognizes the asymmetry and manually identifies and feathers the propeller. V2 also gets pushed up 4 knots from its normal value for the same reason.

I think if you re-read my statement, it is correct....reduced VMCa with autofeather. That being said, you explanation of why there is a difference is interesting as I always wondered why VMCa would somehow change.

That also being said, I don't remember any V1 or V2 in the Twin Otter(It is not above 12,500 pounds MTOW) and rotation seemed to be as much by feel as numbers, especially in offstrip ops. Perhaps there was a liftoff speed on hard surface ops. Is this something new on the -400 series or is my memory fading for the V stuff.

[PanEuropean]

I think if you re-read my statement, it is correct....reduced VMCa with autofeather. That being said, you explanation of why there is a difference is interesting as I always wondered why VMCa would somehow change.

Hi Pelmet:

Attached below is an image of the limitations section of the supplement to the Twin Otter AFM dealing with operation when the autofeather system is inoperative. The supplement is identical for both the Series 300 and Series 400.

That also being said, I don't remember any V1 or V2 in the Twin Otter(It is not above 12,500 pounds MTOW)... Is this something new on the -400 series or is my memory fading for the V stuff.

The Series 300 Twin Otter was originally certified in 1967 under CAR 3, which didn't address or require V1, Vr, or V2, nor did it address or require accelerate-stop distances. Only a 'liftoff' speed based on a margin above stall speed was published. Later, a supplement was published for the benefit of operators who needed to comply with SFAR 23, and that supplement contained V1, Vr, and V2 speeds, as well as accelerate-stop distances. Minimum Vr was established based on a specified margin above Vmca. Depending on the regulatory regime, both the 300 and 400 Series can be operated using either CAR 3 or SFAR 23 certification criteria.

Even later on, Supplement 36 was published to address the needs of Australian commercial operators who had to comply with unique Australian performance calculation rules applicable to Transport Category operations in that country. Finally, around 2012, supplement 37 was published to meet the needs of operators who needed to comply with EU-OPS1, CAR 704 or FAR 121 regulations. Both supplement 36 & 37 apply equally to the 300 and 400 Series.

The takeoff speeds in the various supplements didn't vary greatly from the original CAR 3 certification - the biggest difference arose from calculating a rotation speed based on a prescribed amount above Vmca, rather than a liftoff speed based on a prescribed amount above stall speed. At gross weight, there was no difference, but at weights less than 12,000 pounds, minimum rotation speed was constrained by the need to keep Vr a specified margin above Vma. Of course, the aircraft itself could not read the manual , which meant that it would be very difficult to keep an empty, lightweight Twin Otter on the ground until reaching the minimum 70 KIAS V1 & Vr speed set out in these supplements.

Supplement 37, the EU-OPS1 supplement, also introduced some new performance charts such as brake kinetic energy requirements. None of the new performance charts had the effect of imposing any significant operational limits on the aircraft, but EU-OPS1 regulations required that the additional performance information be provided to operators.

[PanEuropean]

...there is one instance of [Twin Otter] reduced T/O weight and I believe that's structural at colder than -40 due to landing gear. Maybe that's changed as well.

I have not read or heard of such a restriction before, nor am I aware of technical bulletins or operator advisories about such a landing weight restriction - and I'm the person who re-wrote the Twin Otter Series 300 & 400 AFM & all the supplements back in 2011. It is possible that one individual operator may have established their own in-house restriction on landing weight at extremely low temperatures.

[pelmet]

That also being said, I don't remember any V1 or V2 in the Twin Otter(It is not above 12,500 pounds MTOW)... Is this something new on the -400 series or is my memory fading for the V stuff.

The Series 300 Twin Otter was originally certified in 1967 under CAR 3, which didn't address or require V1, Vr, or V2, nor did it address or require accelerate-stop distances. Only a 'liftoff' speed based on a margin above stall speed was published. Later, a supplement was published for the benefit of operators who needed to comply with SFAR 23, and that supplement contained V1, Vr, and V2 speeds, as well as accelerate-stop distances. Minimum Vr was established based on a specified margin above Vmca. Depending on the regulatory regime, both the 300 and 400 Series can be operated using either CAR 3 or SFAR 23 certification criteria.

Thanks for the detailed info. I wonder if any Canadian operator uses SFAR 23 with V-speeds.

[J31]

...there is one instance of [Twin Otter] reduced T/O weight and I believe that's structural at colder than -40 due to landing gear. Maybe that's changed as well.

I have not read or heard of such a restriction before, nor am I aware of technical bulletins or operator advisories about such a landing weight restriction - and I'm the person who re-wrote the Twin Otter Series 300 & 400 AFM & all the supplements back in 2011. It is possible that one individual operator may have established their own in-house restriction on landing weight at extremely low temperatures.

Landing weight temperature restrictions on 100/200 series depending on the mod status.
https://fas.org/man/dod-101/sys/ac/docs/a9ea.pdf

DHC-6-100
Maximum weights With Mod. 6/1020 - "Fuselage Beam, Front Wing Spar Reinforcing"
Landplane (lb.) Skiplane (lb.) Floatplane (lb.)
(With Item 201(a)&(b)) (With Item 202(a)&(b))
Take-off 11579 11579 11600
Landing 11400* 11400* 11600

*See NOTE 5 - Temperature Limitations
Without Mod. 6/1020 - Same as Model 1.

Note 5. The landing weight is 11400 lb. if the airport temperature at which the landing is to be made is at
or above -20°F (-29°C). If the airport temperature is below -20°F, then the landing weight is
restricted to 11000 lb.


Landing weight temperature restrictions on 300 series depending on tire pressure?

Maximum weights Landplane (lb.) Floatplane (lb.) Skiplane (lb.)
(With Item 201(a) or (b)) (With Item 202(a)
Take-off 12500 12500 12500
Landing 12300* 12500 12300*
* Main Wheel Tire Pressure 38 p.s.i.g.
(Below -20°F) 34 p.s.i.g.

I'm not an expert on the Twin Otter and these restrictions may have been removed with SB's.

[PanEuropean]

Hi J31:

Wow, very interesting. I had never seen that before, because I only worked on the Series 300 and 400 aircraft manuals.

The information you cited made reference to a "Model 1", which is actually the "Series 1" - that consisted of the first 5 or 6 aircraft made. When I retired in 2013, only 1 of those was still in service, I think it was on floats down in Peru but I can't recall for sure.

The tire pressure restriction on the Series 300 has an interesting - in some ways, funny - application in real life. If you fill the tires to 38 PSI at ISA, by the time they cold-soak to a temperature less than -20°F, they will be below 34 PSI without any intervention necessary.

[J31]

At the risk of hijacking the auto feather thread, but!

The temperature landing weight restrictions are on Series 1, 100, and 200. Made for poor "legal" landing numbers on short hauls! Did not stop many operators running a bit overweight

I stand corrected on the tire pressures and temperature on the 300. I see the landing weight on the 300 is 12,300 lbs. regardless of temperature. Except on floats it is 12,500 lbs.

[J31]

Was "Auto Feather" a option or mod on the DHC-6 100, 200 and 300? All the 100, 200, and 300's I flew did NOT have "Auto Feather".

[FL030]

Mod on the 100/200, standard on the 300 I believe.