DHC-6 Questions:
 

These two have come up recently and I thought I would ask around the pilot community:

1. For this one assume that you have obstacles in the approach path so that you would normally use full flaps. With a cross-wind would you use less flaps or would you tend to stay with full flaps?

2. On departure would you go for the book procedure of initial climb at 80 knots with flap 10, even assuming no obstacles, or would you use a higher speed? If so, how much higher?

I have my own way of operating as do most people so that I thought I would put these two out there to see what people do with this machine in the real world. Most of my experience is not on this type, besides which I just came back to it early last year.

It gets interesting when human nature comes into it, I find. Some people have told me that they find this or that procedure "uncomfortable." Is it that most modern airplanes tend to fly about the same, making the DHC-6 the odd one out? Anyway, I had a few things I took as givens but it would seem not everyone sees things that way, hence the questions.

Thanks in advance for whatever answers you all can give to these questions along with the reasoning behind them if you like.

Questions
 

Long time since I last flew the twotter, but here goes:

1: Two big variables to take into account. Runway length and wind strength.

If the runway is long enough, about 800mts? flap 30 should do. if it is really short, 400mts, you need full flap to bring the approach speed as low as you can. You really need to have a handle on your x-wind tecnique however to ensure you are on centerline and on the numbers. ( In my experience, this is not for the in-experienced if you are dropping it in over trees on a short runway.) If you cannot maintain centerline with x-controls, go around and wait for the wind to decrease.

2: I always climbed at 95kts no matter what. (This was a -300 and it seemed to give the best compromise for all weights and configs.

Ok, Flak jacket, helmet and heading for the door........

All makes sense Sir Oasis, and you right a strong x-wind in a Twott is not for sissies!

1.) How long is the landing distance available? If you have enough space use less flap. If you're limited in length use full flap and land wing down on one wheel to avoid the wing lifting, a la DC3, then place the other main wheel on the strip and begin your normal deceleration/braking once you have weight on both mains.

2.) 82 kias + 5 to 10 on the climb out with flap 10. 103kias with flap 0 as per the book. Still get good climb performance and have the added speed to give you some leeway with that built in headwind in case of an unscheduled run down. In case of obstacles that need to be cleared I use 82kts best rate. Haven't yet had to clear something that required best angle of climb.

2c worth but I'll give you a discount for cash.

1) I cannot imagine an obstacle that would be so high that it alone would require full flaps, as opposed to 20° flap. Landing with full flap is normally only required when the runway length is extremely short. With flap 20°, AFM reference speed (SFAR 23) and conventional technique, you can easily fly an approach gradient of 10°.

Having said that, the Twin Otter is capable of landing on very short runways, or landing in very strong crosswinds, but it cannot be safely landed on a very short runway in a very strong crosswind. There are many accident reports in the archive that attest to that. The two conditions (full flap landings, and very strong crosswinds) are mutually exclusive.

2) Under all conditions, without any exception, you want to use 10° of flap and target 80 KIAS for the initial climb to 400 feet (or higher if required for obstacle clearance) prior to flap retraction. There is no value, none whatsoever, in using a higher initial climb speed. The best rate of climb for the aircraft is 80 KIAS when the wing is in the flap 10° configuration. That is true regardless of whether you have two engines operating, one engine operating, or whether your Twin Otter is being towed on a rope like a glider.

By targeting and maintaining 80 KIAS after takeoff, you put the greatest distance between yourself and the ground in both the shortest period of time and the shortest horizontal distance covered. That is the universally accepted objective for twin engine operational practice following takeoff - just watch any Boeing or Airbus twin depart from your local big airport, and see if they choose best rate of climb as V2, or if they elect to lower the nose and go faster. They all put the nose way up there and go for the Vy that applies to their departure configuration.

There is no benefit of any kind in flying faster than 80 KIAS immediately after takeoff in a Twin Otter. If an engine fails, you want height above ground (potential energy) in your back pocket, not excessive airspeed (kinetic energy). Excess height above ground keeps you alive, excess speed simply creates a bigger crater when the aircraft hits the ground.

If an engine does fail, all you have to do to maintain the 80 KIAS that you had a few moments ago with two engines operating is to lower the nose of the aircraft to half the pitch angle you were using when you had two engines operating. In other words, if your initial pitch angle was +10° to maintain 80 KIAS with two engines, you stuff the nose down to +5° pitch angle when you lose one engine, and you will then continue to maintain 80 KIAS. Half the engines? Use half the pitch angle, it's simple.

I hate to write a post in the definitive tense, because it can sound arrogant. But in this particular case, given that I have well over 5,000 hours of DHC-6 simulator instruction time (averaging probably 2 engine failure per hour, not to mention half a dozen Vr cuts for every student), and given that I am the author of the AFM, I am being definitive.

Takeoff with flaps 10° (and full calculated takeoff power, by the way - there aren't any Luddites still out there who are using 90% power for takeoff, are there?), rotate to a pitch attitude that will give you 80 KIAS, maintain that speed until no less than 400 feet before you begin to retract the flaps, let the flaps retract fully before you make any power adjustment (the aircraft subsides a bit during flap retraction), transition to 100 KIAS during the flap retraction, and you will enjoy a long, safe, happy life flying Twin Otters.

Michael

One minor correction - don't forget that the colour code markings on the Standard Series 300 airspeed indicator (aircraft without S.O.O. 6093 or S.O.O. 6120) are painted at the CAS number, not the IAS number. It makes no sense, but it was the way things were done under the CAR 3 certification rules that applied to the legacy aircraft. Thus, if you want to obtain Vyse (82 knots CAS), you have to put the needle over the 80 knot IAS position, not over top of the blueline.

By the same token, the redline is marked at 170 knots, which represents 170 knots CAS. If you actually let the needle rise up to 170 on the dial during descent, you have to throw the aircraft away when you land :) because 170 knots CAS is achieved at 166 knots IAS.

The new 400 won't have any of these problems - the speed you see displayed on the screen as IAS will always be CAS as well, because there will be a pitot and static system error correction lookup table embedded in the system software.

Michael

Climb speed
 

Just a correction to my post,

V1. you are correct

I do agree that every take off was made with 10 deg flap and initial speed of 80kts. ( I did not read the question properly, RTFQ). Once flap was retracted and safely away from the ground and obstacles, I accelerated to about 95 kts.

Sir O

Hola V1... When I saw this post I just knew that I would find you here!:ok:
After some South Sudan time I can only back up what V1 says.
The book was written in order for you to operate the aircraft safely and efficiently.
As far as take off's we have the speeds for the appropriate weights from the AFM and those are flown:8 knowing pilots...one can say sometimes:E:E
Landings in the Twotter are some of the more interesting aspects of flying this able aircraft.
The runway surface/length will dictate the flaps used for landings. I don't recall flaps 30 being a normal landing flap, usually it is either 20 or 37.5 as per the AFM. The caution with 37.5 is directional control AFTER you get it on the ground:}
I have been presented twice with crosswinds that were such that I could not land with full flaps, just couldn't get it on the ground:eek:
Option A was a long runway so an eventual landing/arrival was made with Flaps 10 and a strange angle on the horizon:mad:
Option B cannot be talked about here otherwise V1 will be visiting me sooner than I expect him:E
Suffice to say droping over the trees into 400m with a strong croswind will probably require some clothing changes:} the first time you do it but if you manage it the first cold one at base is going to taste real good:D
Happy flying in this unique member of the dehavilland family

Hey Rat Catcher... how much x wind was that? Sounds serious! :) Maybe you should come on my ndege one of these days and I will show you how its done! HAHAHA (but only on tarmac - I cant do dirt these days!)
see you soon!

No - never show folks "how it is done" - sometimes the ending does not turn out as was foreseen, as you can see in this video with an unexpectedly tragic ending. I can only presume that the photographer was taking the video because someone promised to "show them how it was done".

The aircraft was relatively lightweight. It appears from the video that they did not attempt to target 80 KIAS for initial climb following rotation.

Twin Otter Takeoff

Dont worry V1. private joke between me and Rat Catcher. Sadly my 'bush' flying days are over. All by the book now!
:ok:

Ah, OK, missed that nuance, sorry.

Good to see you on Small Fry! VI is an old pal:ok:
Food for thought about your "bush" flying..perhaps when "George" retires, you'll get to fly him!:E:E:E:}

Just because George might take a 'break' - it is still by the book... Quick Data recorders and Flight Data Monitoring these days.... :} Who knows, you might have to behave soon too!

What happened?
 

Where and when was that crash, V1? Is there a report that refers. It looks as though they just banked right after rotation and dragged the wing, but why?

Small Fry...I am wounded:{ I always behave:ok: Perhaps we should not get into the many definitions of that word though:E:E:E:}
See you soon my friend:ok:

Interested on what others in the industry do,

Starts on the -300, I've seen all three methods done and each said to be the best way;

Do you start in feather, full fine, or push to full fine after introducing fuel?

Twotter starts........
 

You start in feather and do not exceed 10 lbs of tq in feather. per the book.

Hi Spad:

Uh, you might have an out-of-date book, or perhaps a Series 100 or 200 book.

For the Series 300, in principle, the engines are started in feather. The exception to that rule is when the air is considerably denser than normal, which is defined as temperatures below 50° F (10°C) and pressure altitudes less than 3,000 feet. When those two conditions exist, the propeller levers should be moved to the MAX RPM (full forward) position prior to engine start.

When the air is quite dense, the engine may hang up on 'minimum flow' fuel (thus yielding about 48% Ng) after start if it is started with the propellers in feather. This is undesirable; the goal is to always have the engine idling on 'governing' fuel (the FCU being the fuel governor) at the end of the start if governing fuel is > minimum flow. The governor is rigged to idle at about 51 to 52% Ng at ISA. At high elevations, the engine will idle faster than that, but when that happens, it is idling on minimum flow, not the governor.

There is no published limitation for maximum torque when the propellers are in feather. I have seen references in some training publications that suggest that 17 PSI torque should not be exceeded (on the -27 engine) when the propeller is in feather, but I have been unable to substantiate these references with PWC, the engine manufacturer. Certainly there is no harm in increasing gas generator speed sufficient to bring a generator on line (idle +15%) with the propeller in feather. I cannot imagine any circumstance that would require Ng greater than that when the propeller is feathered.

Michael (the guy who writes the book... :) )

Be-18 ideas for Twin Otter
 

Who says one has to maintain centerline? With over 4,000+ hours in freight Beech 18s there are many ways to counter crosswinds. Upwind throttle is one way, landing crossways to the runway into the wind is another. I once landed a Beech 18 in a crosswind so fierce that I landed perpendicular (across) the runway. This was in a blinding blizzard in the Chicago area. Taking off from the same airport later, I had to use full throttle on the upwind motor & about 30% power on the downwind motor until I have enough controllable airflow over the rudders to go to mostly full power on both. You're flying a Twotter, not a 777. Use it like a truck & go home to fly another day. :ugh:

Yes there is a published...
 

limit for tq in feather. It is ten pounds and it is to prevent bending the propeller blades. Says so. Try reading your SBs, because I think it is there also but I don't have it in front of me at the moment. I do have an "old book", but it is for a 300, but it‘s not in front of me either.

You do not get 51% +1-0 until you take it out of feather or its rigged wrong. I don't like sending oil pressure to change the propeller pitch until the engine is at least stabilized after start. I prefer all that oil pressure for the engine and gearbox during start.

However I don't do this anymore and I would have to refer down to ops or maint for any changes. I do appreciate your info.

Uh, thanks for your 'Word in God's Ear', as the German language saying goes. :E

I think you might be referring to something that may have been published 30 or 40 years ago in the TABs (Technical Advisory Bulletins), which were a DH specific predecessor to Service Information Letters. Those documents are informal. They are not controlled, nor is a revision service (in the sense of how SBs and ADs are revised) provided.

The AFM, on the other hand, is formal, it is the only document related to the aircraft that is approved by both the regulatory authority of the State of manufacture and the regulatory authority of the State of registration.

The propeller governor has its own oil pump, therefore, there is no pressure loss when oil is routed to the propeller. There is a decrease in the volume of oil, but this is of no consequence during the start process, because all of the oil should be back in the tank at the beginning of engine start.

I assure you, there is no limitation on torque when in feather published in the AFM.

I disagree with your comment about not getting idle on the governing fuel flow prior to taking it out of feather. Many (but not all) of the PT6A-27 engines installed on the Twin Otter will idle on minimum flow at ISA pressure altitude until the propeller is taken out of feather. This is an unintended consequence of Py air being bled at the Nf governor when the propeller is feathered. But, not all engines have this characteristic, nor is this characteristic by design.

Michael

All this technical talk is killing my ears!
 

Hej guys, I have seen a Twin Otter flown in almost every condition imaginable... Dhc-6 300 full flap t/o in 50 knots of wind, empty on both floats, wheels, and also skis. If the rnwy is short, land with full flaps and c/l max, if you run out of rudder use differential power to straighten the nose with centerline... I have had success with that method on floats, and wheels... with skis I always landed into wind if I could. Don't touch the tiller bar until you are finished flying the aircraft... I once saw an RCMP Twin Otter go all over the rnwy after touchdown because the pilot started to play with the tiller too early in a heavy crosswind - he went right off the rnwy and "staition 60'd" the nose gear. On floats, normally pitch locks are installed to keep the props in fine pitch so you have immediate thrust available after engine start... Use the doors and keep them closed... I once had to ferry a machine back to base empty because one of the engineers took the copilot's door off to repair something and another guy thought it needed to go in for maintenance and stuck it on the next flight out... not sure if there was as supplement for that :E.... but it sure was noisy and drafty. If you have good skills you can use the Otter like the work horse it is... if you don't and you start to push the limits, you can get yourself into trouble very quickly...
Fly safe brethern... and enjoy the Otter! :}
6to8

Thanks again V1....unfortunately
 

I’m afraid this German colloquialism went completely over my head. I have no idea what it means.
Thanks for the info. It’s certainly worth considering. We ordered new manuals from Bombardier, Pratt, and, Hartzell in 2001. Along with all the technical publications available, we also ordered and maintain subscriptions and revisions for the manuals, SB’s, SI’s ect.. Among the Component Overhaul Manuals were the Technical Advisory Bulletins, (TABS). If information were published and has not been amended, superseded, or deleted, then it is applicable IMO. With this and other manufacturer’s published information in mind, I amended the AFM to include limits I deem in the interest of safety, or, otherwise in our institute’s interest. We have made several other changes or additions to the AFM as well. Just as we do the Maintenance program as may be applicable.
As always, I appreciate your insight.

Guys, trust me on this one (fer' Pete's sake, I'm the person who writes the AFM): There is no published limitation for torque when the propeller is feathered.

Plain old COMMON SENSE suggests that you don't want to apply more power to a feathered propeller that you would need to achieve idle +15%. Let that be your guide.

Michael

Well I was thinking....
 

that if Viking has the legal authority to amend, revise, or, delete the AFM and Maintenance Manuals for the DHC6-300 Twin Otter built by deHavilland, then it would seem plausible to me that Viking can publish those amendments, revisions, and, deletions.
If they have, then I'm sure we have them on file. If we have them on file then they are being adhered too. Making all this redundant. As I said before, I'm no longer directly involved with Operations and Maintenance. However I am curious as to why anyone would find it necessary or convenient to apply so much force against flat paddles spinning near their heads.
We alter our airframes extensively and have several configured alterations for the Twin Otter so I’ll see if I can bother our engineering department with the question. Thanks again V1.

Viking holds the Type Certificates for the DHC-1 through DH-7 aircraft - Viking took over responsibility for these TCs from Bombardier in 2005, and has been publishing all the documents (AFMs, MX manuals, etc.) for these aircraft since then.

Michael

Thanks V1....
 

...but yes I know this already.

Thought the best Angle of climb (altitude gain per unit ground distance) was 87 kts.
so Its not "rate of climb'' we should be talking about.
Vx is always lower than Vy and Climbing at Vy allows pilots to maximize the altitude gain per unit time.
So for obstacle clearance Vx would always be the the speed we concerned with....

Back to the bunker...fire away:}

See the elaboration of that in my post (above) of 26th May 2008, 11:30.

Michael

With the prop gov having its own pump, is there a reason to start in feather rather than full fine all the time?

Our twin otters operate in a tropical environment and also idle in feather at about 48% Ng and require full fine for ballpark 52% Ng.

Engine start in fine pitch
 

Smitty, we used to start up with the props in fine pitch only because pitch locks were installed and it was used to bring immediate thrust up while on floats. You don't want to have foward lurch on a closely parked dock when you bring the prop out of feather....
Others may know more.... I just drove the "Dienst Pferd" (Work Horse)!
Cheers
6to8

The prop gov has it's own pump....
 

It don't start pumping till the engine starts turning.....and the "pump's" speed, (thus it's efficiency), is directly related to engine speed up to idle.
Think about that for a second or two.

PT6A-27 48% NG idle in feather, with 52% NG in full fine sounds quite normal to me and that is what they taught at Flight Saftey. It is what is in the book also, (unless it has been changed recently).

I'm not trying to hijack this thread, but I've posted this question elsewhere with little luck. This thread is obviously active with folks that know there away around a Twin Otter.

I'm looking for operating costs for a Twin Otter on amphibs (300 series with -27 engines). Salt water ops.

Real world hourly operating costs would be very helpful! Any advice for someone looking to purchase and operate one (I know this last question is vague)? Part 135. I'm trying to run the numbers as a very early start to a business plan.

Thanks in advance!

Twin Otter Inprovements
 

Would love to see the exhaust system on the DHC-6 400 improved or modified not to blow directly over the leading edge of the wing. Creates lots of mess and heat and in time corrosion. Maybe a Dash 7 design would be better. Also landing lights should be positioned on outside of the wing fence to get away from the exhaust gasses. Pilot's seat needs better leg support, just not long enough. On long trips, ferries etc very uncomfortable. And hopfully those dreadful fuel pumps will be more reliable... Just a thought.

Reebs
 

Very Good Ideas and not to difficult to bring about either. As well, I for one would like to be able to order new Twotters with any of the old deHavilland modifications, such as the camera bay, wing hard points, and, 250 amp electrical system. Even a higher amperage electrical system than 250A would be more desirable. It would also be nice if the fuel tanks were integral rather than bladder.

New Twin Otters CAN be ordered with any of the previously available de Havilland special purpose modifications (cameras, scientific survey, etc.).

Why anyone would want bigger generators is beyond me. The Series 400 aircraft only consumes about 50 amps of power with all avionics working, and it already has two 200 amp generators installed. One generator alone is more than sufficient to support every electrical load operating at the same time, including full de-ice with the heated windshield, hot props, every single light bulb in the aircraft turned on, etc.

The new avionics in the aircraft draw far, far less power than the old avionics, and all of the lighting (excepting the two wing inspection lights) is either LED or high intensity xenon discharge.

Michael

V1
 

Research aircraft Michael. To power more research payload. More power available, means a more valuable aircraft. NOAA, NSF, NRL, British Arctic Survey etc..

NSF recently bought a new G-5 with 30 million in added structural modifications to add to their research aircraft fleet. How many Twin Otters is that? For low altitude research, slow airspeed means higher particle resolution. The Twin Otter has proven itself extremely valuable in terms of payload utility, reliability, and, operational cost efficiency, though a little lacking in mission endurance or loiter time.

More wattage available means more science equipment. More science equipment means more missions. More missions means MO MONEY.

We all know what mo money means.

Well, there are 350 amps of additional (surplus to needs) electrical power available now on every Series 400 aircraft produced. If a customer needs more power than that (perhaps they plan to run an electric-arc blast furnace installation in flight) :) , we can provide higher amperage generators, because this is consistent with what I said earlier: All of the "special options" that de Havilland ever offered - this includes additional electrical capacity - are available from Viking on new build aircraft.

Seriously, though, electrical power needs on aircraft (of all types) to run comparable types of equipment have been steadily declining over the past 30 years. My guess is that an aircraft fitted with survey, photographic, or research equipment today would require far less electrical power than an aircraft fitted with similar capability equipment 30 years ago.

Michael

V1.....
 

The more power the better Michael.