View Full Version : DHC-6 Questions:

26th May 2008, 17:06
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.

Sir Osis of the river
26th May 2008, 17:37
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........

26th May 2008, 17:47
All makes sense Sir Oasis, and you right a strong x-wind in a Twott is not for sissies!

Solid Rust Twotter
26th May 2008, 18:00
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.

V1... Ooops
26th May 2008, 19:30
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.


V1... Ooops
26th May 2008, 19:45
...82 KIAS ... on the climbout with flap 10°

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.


Sir Osis of the river
27th May 2008, 04:14
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

Rat Catcher
27th May 2008, 12:54
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

27th May 2008, 23:52
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!

V1... Ooops
28th May 2008, 01:52
...come on my ndege one of these days and I will show you how its done!

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 (http://www.youtube.com/watch?v=QBawotesGdg)

28th May 2008, 08:04
Dont worry V1. private joke between me and Rat Catcher. Sadly my 'bush' flying days are over. All by the book now!

V1... Ooops
28th May 2008, 08:20
Ah, OK, missed that nuance, sorry.

Rat Catcher
28th May 2008, 09:01
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:}

28th May 2008, 09:57
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!

28th May 2008, 17:30
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?

Rat Catcher
30th May 2008, 06:09
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:

Lando Calrissian
25th Apr 2010, 11:23
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?

25th Apr 2010, 12:02
You start in feather and do not exceed 10 lbs of tq in feather. per the book.

V1... Ooops
25th Apr 2010, 20:39
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... :) )

25th Apr 2010, 20:44
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:

26th Apr 2010, 00:27
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.

V1... Ooops
26th Apr 2010, 01:23
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.


26th Apr 2010, 20:08
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! :}

26th Apr 2010, 22:49
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.

V1... Ooops
27th Apr 2010, 12:08
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.


27th Apr 2010, 13:04
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.

V1... Ooops
27th Apr 2010, 19:10
...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.

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.


27th Apr 2010, 22:27
...but yes I know this already.

28th Apr 2010, 10:40
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:}

V1... Ooops
29th Apr 2010, 00:53
...So for obstacle clearance Vx would always be the the speed we concerned with....

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


3rd May 2010, 10:57
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.

3rd May 2010, 12:36
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)!

3rd May 2010, 12:50
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).

5th May 2010, 02:13
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!

6th May 2010, 21:46
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.

6th May 2010, 23:42
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.

V1... Ooops
7th May 2010, 00:00
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.


7th May 2010, 19:21
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.

V1... Ooops
7th May 2010, 19:58
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.


7th May 2010, 23:02
The more power the better Michael.

8th May 2010, 07:02
I remember the grand kerfuffle when I wanted a cigar lighter socket in a Twotter. Not to smoke cigars but just to run my noise-cancelling headset, one of the early ones that didn't have a battery pack. There I was thinking to just find a source of juice and tap into it like some bush man. Wrong! You needed technical approval and blah-blah-blah... (Meanwhile, for all you Cessna drivers out there that company got a new anti-smoking chief exec who did away with same on his products, obviously not thinking of other things you could power off a cigar lighter!)

Here you already have the bog-standard PT6 starter-generator approved and in place. If you wanted to go for more amperes I guess that would mean finding a new, more powerful starter-generator and getting that certified. Then you would have a problem with spares, some aircraft with the old kit and one or two with the new kit and how do you get your overhauled unit when it's a one-off and you are out back of nowhere and, "Oh, look, they sent us an old model starter-generator instead of the new model," or vice-versa...

I already saw a Chief Pilot staring a hole in the mahogany bar counter after some bright spark had sent us a PT6-28 instead of a -27 because it's thermally adequate, not thinking about that funny gizmo on the nose case that only the Twotter has. (Of course the old engine had been flown to time-ex on Friday afternoon for a quick engine change on the weekend to be back in service on Monday.) I slithered up next to this unfortunate to ask, "So, Pat... Heard any good PT6 jokes lately?" when he did rather exhibit signs of a serious sense of humour failure there, yes.

I think much of the appeal of the Twotter is in its rugged simplicity (who would buy it for its looks, a blind man?) and any upgrades have to detract from that.

V1... Ooops
8th May 2010, 11:43
...I wanted a cigar lighter socket in a Twotter.

My goodness, we could not put a 'cigar lighter' in an airplane today, we would get sued to death for contributing to lung cancer. Heck, it's all but illegal to smoke a cigarette in North America these days, let alone a cigar. I fully expect that within the next 1 to 2 years, the government here will ban possession of tobacco.

But... we have provided two 14 volt convenience outlets as standard equipment on every Series 400 Twin Otter. One is located on each side of the center pedestal - about beside where the radar screen used to go on the legacy aircraft.

14 Volt Outlets

They are provided to charge up your phone (or whatever). You won't need them to power a noise cancelling headset, because two David Clark noise cancelling headsets, permanently hardwired to and powered from the aircraft, are also provided as standard equipment. But, I suppose if you really want to use your own personal noise-cancelling headset (and not the ones that are basic to the aircraft), you could power it from these outlets.


V1... Ooops
8th May 2010, 11:47
...not thinking about that funny gizmo on the nose case that only the Twotter has...

Do you mean the beta backup valve? That system has been deleted from the Series 400 Twin Otter. You still get annunication when the propellers are in the ground fine mode (the Np indicators change to a cyan background), but that whole beta backup system is gone.


8th May 2010, 17:26
If you were a cruel person then you would laugh at the sight of those guys unwrapping their freshly-overhauled PT6 and then freaking out because... no beta back-up valve! I am and I did, of course!

So this first airplane, is it being delivered with the uprated engines or bog-standard -27s for, uh, commonality? A little bird landed on my shoulder, you see...

Also, could some perverted nicotine fiend stick a cigar lighter into that "utility socket" and then use it to light a cigarette, or would that void the factory warranty?

9th May 2010, 00:13
We are forced to wear the David Clark Head Vise?

That will be the first thing we rip out then.

9th May 2010, 00:21
We don't need "technical approval". We don't care about warranties and we don't need Approved Data, Supplemental Type Certificate or anything from the FAA. We have our own engineering.

...and yes, I have your inquiry bookmarked for future reference.

V1... Ooops
9th May 2010, 02:46
...We are forced to wear the David Clark headset?

No, there is no obligation to wear the headsets that are supplied with the aircraft. The David Clark noise cancelling headsets (their top of the line model) connect to a David Clark fitting that is installed behind each pilot seat, where the standard headset fitting is on the legacy aircraft. Incorporated into that fitting are two standard phone receptacles that allow a conventional headset to be used. The receptacle design is a mechanical interference fit, in other words, either the David Clark noise-cancelling headset can be connected, or a standard two-plug headset can be connected, but it is not possible to connect both at the same time.

We elected to provide the two noise-cancelling headsets as standard equipment on every plane for occupational health and safety reasons. The flight compartment of a Twin Otter can be quite noisy at takeoff power, and the superior noise attenuation of these headsets (combined with the noise cancelling capability) protects the pilots from exposure to excessive sound pressure levels.

...We don't need "technical approval". We don't care about warranties and we don't need Approved Data, Supplemental Type Certificate or anything from the FAA. We have our own engineering.

Uh, just out of curiosity... who is 'We'? :)

9th May 2010, 06:26
After that your head has been reshaped to fit the David Clark headset, sort of like those Flathead Indians but in a slightly different way.

I now have the latest David Clark, the X-11. It is very light and comfortable, if not quite as gee-whiz as the Bose. When you are a clumsy person who works in difficult environments (heat, dust, humidity) then a rugged headset is a good thing and I always found David Clarks to be tough. The latest one seems to have kept the virtues while also being much more comfortable.

I wear custom-fit earplugs along with my headset, something I should have started with a long time ago, with hindsight. Nowadays the wife asks, "Aren't you going to answer the phone?" when I can only reply, "Phone?" Aaah, DeHavilland!

Must be the Mexican Police. "Engineering approval? We are the Mexican Police. We don' need no steenking Engineering Approval!"

9th May 2010, 23:17
ref: AC 00-1.1 Chap 1 Par 2d. (5)

9th May 2010, 23:51
you are right.

We are not our southern neighbor's mountain police, but we don't need no stinking badges.

V1... Ooops
10th May 2010, 02:36
...I now have the latest David Clark, the X-11. It is very light and comfortable...

We're installing the H10-13.4 as standard equipment, along with the panel mount power module, which is located behind the pilots in the same places as the receptacles have historically been.

I've used this headset for all the hours I have flown the Series 400, and I'm quite happy with it. We have invited about a dozen pilots from the industry to fly the Series 400 as part of our human factors evaluation, and no-one has had anything to say about the headset, other than commenting that it is nice to have a noise-cancelling headset provided as standard equipment.


10th May 2010, 07:37
Do you still need to press the intercom button to talk to each other? I laughed at myself one day, having a conversation with my cojo whilst shut down and without headset on the ground. I discovered I was still pressing the intercom button everytime I said something. Doh!

I guess thats what 120hrs plus per month in the plane does!

V1... Ooops
10th May 2010, 10:45
I know exactly how you feel... :)

The intercom on the Series 400 Twin Otter is voice activated. I was pretty sceptical about this idea when I heard that this was the way that the Honeywell KMA-29 audio panel worked, but after having flown the aircraft for a few hundred hours, I am very happy with it.

The electronics behind the voice activated intercom are very fast. The intercom activates right away when you speak, and there is no clipping of the first or last words of what you say. I was a bit concerned that the intercom might activate when not wanted during (for example) takeoff, but it doesn't do that. In fact, it doesn't even activate if you open the window, unless you start to speak.

The rocker switch on the control yoke only moves in one direction, that is from the center to the 'radio transmit' position. It's blocked from moving in the other direction, which means that after a few days in the plane, you get used to not pushing it automatically when you want to speak to the other pilot.

It's particularly nice for the right-seater to have the voice-activated intercom during busy times, for example, when reducing power just after takeoff, etc.


10th May 2010, 11:22
I doubt very seriously that anybody will decide whether or not to order a new -400 based on the headset type. Sales rhetoric can sometimes paint an invincible picture of product. Every David Clark I ever put on was uncomfortable after an hour or so. I don't have to wear them anymore so I digress.

1st Jun 2010, 08:51
I wonder what the Certification basis will be for the 400. Is Viking applying a STC or re-certifying according to FAR23 ?

V1... Ooops
2nd Jun 2010, 05:10
The Series 400 will be appended to the existing type certificate for the preceding versions (1, 100, 110, 200, 210, 300, 310, 320) of DHC-6 aircraft.

Certification is not my speciality, but to the best of my knowledge, the 400 follows substantially the same cert basis as the 300, except in areas where the aircraft has been modified (for example, the new avionics), in which case the prevailing (most current) certification criteria is applied.

There are some areas in which Viking has voluntarily upgraded the aircraft to a more contemporary cert basis that what is required.

Hope this provides more clarity than confusion... :)

galaxy flyer
2nd Jun 2010, 16:39

Please check your PMs.


2nd Jun 2010, 17:38
What is that old thing, CAR3? You have some really grey areas in that, stuff that just wouldn't be allowed today, so that I doubt it would be economically doable to recerify this machine to the latest standards. There is nothing wrong with that; have a look at what Boeing do with the 737, for instance.

When you get involved with the DC-3, is that ever another world. That thing doesn't even have stall breaker strips and I had Mr Training Captain Ironbutt McHardass telling me that "You young guys (I guess I was only 32 then) don't know how to do a full-breaking stall but show me whatever you can..."

Just a short time later there we were looking at all green in the left front windscreen and all blue in the right front windscreen as we were ploughing out of the sky over the Florida Glades with lots of down and not much forwards, full right aileron but not a lot of change happening in my lousy little life. "So this is why they put stall breaker strips on modern airplanes!" was most of what was going through my tiny mind. No telling what was going through his but the rest of the ride was much quieter! Then you have its ergonomics, from before that word was even thought of, and it shows!

Sit there staring at your new glass panel and be happy, is all I can tell you!

phoq af
5th Jun 2010, 17:53
Hi Mike M

I quite think u r gonna X the Atlatic with the latest of the fleet soon...
Coming back to the initial thread:

I agree that instead of 80kt, Flaps 10, id go 87+flaps 0 to clear obstacles... as I only see 1 and only Vx under DH certif/AFM/etc manuals. Am I wrong?

Moreover, 80kt is Vyse @ 12500lb, so the correct reply should be Vyse for Flaps 10 (ie 80-4/1000lb lighter than MTOW) and best RoC (SE+Flaps10 config). Computed accordingly. Wrong again?

Even you, V1..., I d be surprised if you can confirm 80kt being best angle with flaps10° and BOTH engine at MCT?
Your lights on this being very welcome!


5th Jun 2010, 18:06
Big thing with starting in feather is in the cold, until the oil warms up, you run chance of blowing prop seals

O Molina
5th Jun 2010, 22:02
Hi people, I'm doing a job on seaplanes and I wonder if someone could give me the maximum component of crosswind in water DHC-6 and distances of T/O MTOW and LDG MLW, all distances in water.


O Molina

11th Jun 2010, 19:36
Suggest contact Kenn Borek, they are the biggest Twin Otter operators in the world with floats.. Good Luck

V1... Ooops
4th Aug 2010, 06:22
... instead of 80kt, Flaps 10, id go 87+flaps 0 to clear obstacles... as I only see 1 and only Vx under DH certif/AFM/etc manuals. Am I wrong?

Moreover, 80kt is Vyse @ 12500lb, so the correct reply should be Vyse for Flaps 10 (ie 80-4/1000lb lighter than MTOW) and best RoC (SE+Flaps10 config). Computed accordingly. Wrong again?

It depends on what you are trying to accomplish, and what the circumstances are.

The aircraft achieves best rate of climb when in the flaps 10 configuration at 80 KIAS. It doesn't matter how many engines you have - 0, 1, 2, or 3. The aircraft also achieves best angle of climb (when in the flaps 0 configuration) at 87 KIAS.

If you wanted to fly up the side of the Matterhorn, and you were already established in level flight a few miles away from the base of the mountain, then for sure flaps 0 and 87 KIAS would give you the best angle of climb. That is the configuration I would choose for that task, and that configuration corresponds to the Vx published in the AFM.

On the other hand, if I had a short runway with a tall tree at the end, that means I am going to be configured with flaps 10 the moment I become airborne, because flaps 10 is the only approved takeoff configuration for the DHC-6 Series 300 or 400 landplane. Personally, I would leave the flaps at 10 and target 80 KIAS (best rate of climb for that flap configuration) in order to get over the tree. I would not retract the flaps and target 87 KIAS (best angle of climb), simply because the performance loss during flap retraction would greatly outweigh any performance gain arising from climbing in the flaps 10 / 87 KIAS configuration.


11th Sep 2010, 01:30
Thought I read somewhere that there was a specific reason that the multi crew call for flap retraction after take off was "selected AND running"?

Getting too old...

12th Sep 2010, 16:09
Quotes from V1:
Michael (the guy who writes the book... )

Guys, trust me on this one (fer' Pete's sake, I'm the person who writes the AFM) ....... (ad nauseum) .........

Yes, we get it already. You are the guy who apparently: [has] well over 5,000 hours of DHC-6 simulator instruction time ...

Get some real-world otter time and then maybe people will give you some of the respect that you are so desperately seeking.

My guess is that an aircraft fitted with survey, photographic, or research equipment today would
require ...

I cannot imagine an obstacle that would be so high ...

There is no value, none whatsoever, in using a higher initial climb speed...
Exactly, that is your guess, and you can not imagine, and there is no value that you can think of. No one should let the scope of their own experience, however vast or little, limit what they think is possible out in the rest of the world. There is always something a little beyond what we have experienced for ourselves.

I hate to write a post in the definitive tense, because it can sound arrogant.

Yes, it truly does.

Ah, OK, missed that nuance, sorry...

Lighten up already Mr. Moore, jeez...

V1... Ooops
12th Sep 2010, 21:41
...Exactly, that is your guess, and you can not imagine, and there is no value that you can think of.

No, it's factual. For any given wing (flap setting) configuration, there will be one speed only that gives best rate of climb. In the case of the Twin Otter wing, in combination with a 620 HP limit and flaps 10 extended, best rate of climb will be achieved at 80 KIAS, assuming MTOW. At higher speeds, rate of climb degrades.


20th Sep 2010, 08:55
edited by mods

22nd Sep 2010, 00:28
Trying to PM you, keeps bouncing back.

Please PM for my Pvt email.


V1... Ooops
22nd Sep 2010, 01:20
Hi Rupert:

I'm not sure why the PM feature is not working.

I sent you a PM containing my direct email address.


22nd Sep 2010, 10:34
Thats the first time I've read about not reading the AFM as a point of pride. Not my style, but hey - you're not dead so bonus points to you.... :) I guess the plane is easy to fly!

Meathead Pilot
22nd Sep 2010, 16:04
Slightly off topic here..A friend of mine got hired to fly twotters as a copilot.. He is on a JAR license and wants to know if he can unfreeze his ATPL with twin otter time???? The company operates the airplane multi crew and the law in the particular country requires twotters to be operated 2 crew...

Would appreciate feedback from people who have been in similar position... Thanks in advance..

22nd Sep 2010, 22:40
He should check with his licensing authority. Sometimes they change their minds on this one but if it's required by legislation to be flown two-crew then time as an FO should be okay.

27th Sep 2010, 04:14
Yes, it can be tricky....to log FO time on a US license it has to be an operation that requires two pilots (ie; 135 IFR with 10 or more pax; 121 ops etc) or an aircraft that requires two pilots (in the US, this is not an Otter). The company requiring it doesn't count, only government regs. If the country you are operating in requires it, I think that might do. Call OK City and ask...

If you can't log FO time, then you can always log PIC while sole manipulator of the controls (this doesn't mean you are acting as PIC, just that you are allowed to log it, as two pilots cannot fly the plane at the same time). Many Capt.s won't like this as it implies (correctly) that they cannot log the time that you fly.

V1... Ooops
27th Sep 2010, 17:30
... if it's required by legislation to be flown two-crew then time as an FO should be okay.

This has been my experience as well - if the operating regulations require two pilots, then both pilots can log time appropriatly (PIC/SIC). The operating regulations (2 pilots required) trump the certification regulations (1 pilot required) in this case, because they are the more restrictive regulation.


Meathead Pilot
28th Sep 2010, 04:24
Thanks for the info guys.. Appreciate it.. The regulation in this particular country requires the twin otters to be operated 2 crew since its a commuter operation..

23rd Oct 2010, 06:36
Just wondering,

Why is auto feather armed for take off but not for landing?

V1... Ooops
23rd Oct 2010, 16:16

During the take-off phase of flight, the aircraft has low kinetic energy and low potential energy. Hence the benefit of the autofeather system promptly identifying and feathering an engine following a loss of power.

During the approach and landing phase of flight, the aircraft has both kinetic and potential energy in reserve. An engine failure is less of a problem because the total power needed for approach and landing is less than what one engine alone can produce. For that reason, there is no need to use autofeather during that phase of flight.

There are other technical issues involved (for example, power lever position is steady during take-off, up at the forward end of the range, but variable during approach and landing), however; then issue of energy reserves (both potential and kinetic) is the main reason.


Cardinal Puff
23rd Oct 2010, 16:43
May also not have enough power applied to arm the autofeather system on the approach.

V1... Ooops
23rd Oct 2010, 19:06
...May also not have enough power applied to arm the autofeather system on the approach.

Correct, but that is more an 'effect' rather than a cause. Because the system is designed only for use during take-off, and because the system has to allow a rejected take-off to be carried out without resulting in an autofeather, both power lever position and engine torque pressure are sensed.

The autofeather system will not arm unless torque on BOTH engines is above a specified value (this will vary slightly depending on mod status), and unless the power levers themselves are physically quite far forward. The power lever position is sensed in order to allow deactivation (disarming) of the system during a RTO prior to torque dropping below the torque sensor threshold.

So, even if enough power was carried in the approach to satisfy the torque sensor threshold, it is very unlikely that the power levers would both be far enough forward to arm the system.


23rd Oct 2010, 22:21
I agree with what your saying regarding low power in the approach and landing and as with other aircraft types the system wont arm anyway.

However for the go around you would be in the same position as for a normal take off with low kinetic and potential energy and if selected the autofx system would be armed.

V1... Ooops
29th Nov 2010, 00:25
...the FAA approved MMEL allows flight with the Beta Backup system deactivated, but states that reverse can not be selected. However when reverse is selected by twisiting the power lever grips, it de-activates the beta backup system anyway.

Hello MachTwelve:

There is a difference between "depowering" the beta backup system in accordance with the MMEL you refer to, and the "disarming" of the beta backup system that takes place during normal operations when you twist the power levers.

The nuance in the wording is this: If there is some form of defect with the beta backup system, the MMEL allows you to dispatch (for a limited number of days) with the system turned off. You accomplish this 'turning off' by pulling the circuit breaker for the system, and once this is done, the system does not function at all. The MMEL refers to this process as 'deactivation', in this context, they mean 'depowering'.

When the beta backup system is operating normally, once you twist the power levers and pull them aft of the idle stop (thus actuating the microswitch above the power levers that detects the grips being twisted), the beta backup system DISARMS for each propeller (individually) the moment that each propeller blade move to a position that is finer than +9° blade angle. Once the system has disarmed for one or both blades, it will not re-arm until the propeller blade moves out of the ground fine range, to a blade angle of +11° or more.

The purpose of the DISARMING (note the difference in wording between 'disarming' and 'deactivation') is to ensure that if the pilot rapidly moves the power levers forward - thus changing the state of the grip switch to indicate that the pilot has no longer selected ground fine range - the beta backup system will not begin to function as the propeller blade moves out of the ground fine range, back towards the idle (or coarser) blade angle position.

Consider this scenario: The pilot lands, then hauls the power levers all the way back to passenger seating row 3. The engines and propellers respond, and the propellers move to the full reverse (-15° blade angle) position. The pilot then rapidly shoves the power levers forward to the idle position, thus releasing the grip switch that indicates to the aircraft that the pilot has commanded ground fine range. The propellers will not move back to +11° as quickly as the pilot can move the power levers forward... there will be a delay of one or two seconds before the propellers move to the idle (+11°) position. If there was no disarming relay, the beta backup system would activate and cycle as the propeller(s) moved forward, because the grip switch would indicate that the grips were not twisted, but the propeller microswitch(s) would still indicate that the propellers were at a blade angle less than +9°.

Hopefully that explains the distinction between 'deactivated' (i.e. circuit breaker pulled) and 'disarmed' (a very specific condition governed by Boolean logic that looks at the sequence of activation of the grip switch and the propeller microswitch(s).

...Why can you not select auto-feather prior to descent?

Such an action is inappropriate for the DHC-6 aircraft, which has a different design of propeller control than the Beechcraft that you refer to. There are far too many engineering and certification criteria involved to explain them all here in the discussion forum, but, suffice to say that the subject has been most thoroughly investigated from an engineering point of view (I have read literally hundreds of pages of engineering analysis done when the aircraft was first certified back in the late 1960s), and it is inappropriate, undesirable, and arguably quite unsafe to select autofeather to 'ON' for landing the Twin Otter. The DHC-6 autofeather system is designed and certified for use during takeoff only.


V1... Ooops
1st Dec 2010, 05:44

You ask some very reasonable questions. I'm going to try my best to answer them, however, because it is no secret here in the forum where I work or what I do at work, I am going to have to be somewhat circumspect with my reply.

With the system depowered (via the MEL), why can you not select reverse?

I have no answer for that. That particular restriction has been present in the MMEL since before the Old King died. If I was to rewrite the MMEL today, I would not include that restriction. The beta backup system is only depowered (i.e. circuit breaker pulled) when there is a problem with the (electrical) beta backup system... under these conditions the (mechanical) propeller control system is operating normally. Hence, I cannot explain why that requirement to not select reverse is there.

It is possible that the restriction to not select reverse arises as a result of a certification requirement that states that an aircraft equipped with a propeller that can operate in a ground fine range must be equipped with a system to annunciate when the propeller is, in fact, in the ground fine range. Because this annunciation system (the beta lights you see during normal ground fine operations) is powered from the same CB that you pull to depower the beta backup system, you lose the annunciation of ground fine range when you pull that CB. Hence, because there is no more annunciation of (normal, commanded) ground fine operation if you have pulled the CB, the MMEL then states that you must not invoke an operational mode (ground fine)that you know will not be annunciated to you. That is, however, a guess on my part.

I assume that the Beta Backup system was installed originally to avoid, the risk of inflight prop reversal at high torque settings...

No, not at all. If you think about it for a moment, the only possible way that the propeller can enter the ground fine range (in other words, zero thrust or reverse) is if the propeller is in an underspeed condition relative to what the pilot has selected with the prop levers. It is flat-out impossible for the propeller to enter the ground fine range if the power levers are significantly forward of the idle position (in other words, creating the 'high torque setting' that you refer to).

This is because if you have high torque present when the power levers are in the forward range, the propeller will rotate rapidly, and the propeller governor will reduce oil flow to the propeller hub in order to coarsen blade angle (for the purpose of limiting RPM to what you have selected with the propeller levers).

The above is precisely why the abnormal checklist for a cycling beta light (indicating mechanical failure somewhere, with the beta backup system operating correctly, as it was designed to do) tells you to move the propeller levers to the minimum RPM position - this to make it as easy as possible for the propeller governor to take control - then add a bit of power - that to increase propeller speed, so that the propeller governor becomes active, begins to govern, and restricts oil flow to the hub.

Why... has the FAA approved an MEL for it?

The beta backup system is analogous to a smoke detector in a house. If the smoke detector activates as a result of the presence of smoke, hey, it's doing what it was designed to do, and the 'memory items' on the checklist begin with "Get out of the house..." However, if the smoke detector activates and starts making a great noise and commotion, and subsequent careful investigation shows that there is no smoke anywhere (perhaps you banged it and caused something inside the smoke detector to break), then, the checklist for your house suggests that you depower the smoke alarm and then carry on as normal, taking note that you no longer have a warning system to advise you if smoke is present.

The MMEL entry for the beta backup system addresses circumstances when the backup system operates when it should not operate (usually as a result of breakage of a microswitch out at the propeller). For example, consider that you are flying along in cruise, when suddenly one propeller slowly begins to feather. Torque rises at first (then drops as the propeller blade stalls), and Np drops dramatically from your usual 75% cruise to about 20 or 25%.

You look out the window, and see a propeller that is obviously feathered. Your leg muscles are throbbing from stuffing full rudder in to counteract the complete loss of thrust from the engine with the feathered propeller. But... one little beta light is illuminated, indicating to you that the propeller is in the ground fine range (less than +9° blade angle). You look out the window again and confirm that the propeller is at the full feathered position (+87° blade angle). The warning system - the beta backup system - is obviously crying Wolf, same as your smoke detector did. So, the checklist for the steady beta light tells you to do exactly what you did with your smoke detector - remove the power from the warning system that is raising the false alarm, then, go back to business as usual.

The MMEL entry allows you to dispatch from the next place that you land, and get the plane back to a maintenance base to get the "warning system" fixed.

Does the AFM contain an abnormal to deal with exactly this situation?

Yes. There are two abnormal procedures that deal with the beta backup system. The abnormal procedure for a STEADY beta light explains how to deal with a false alarm such as what I just described above. The abnormal procedure for a FLASHING beta light tells you how to deal with a mechanical failure of the propeller linkage that has caused the propeller to enter the ground fine range without the pilot commanding it to do so.

Note that neither of these procedures are memory items, and neither one requires fast or immediate action.

If you have a FLASHING beta light and you take a long time to complete the checklist, the worst that will happen is that the aircraft will yaw back and forth a little bit every time you reduce the power to a very low level... your passengers will think that you have a new co-pilot who is flying the plane while wearing 10 pound combat boots.

If you have a STEADY beta light and you take a long time to complete the checklist, the worst that will happen is that you will get a leg cramp from pressing on the rudder pedal (assuming that you don't apply trim). What you will have is, effectively, a single engine aircraft resulting from one propeller feathering itself in flight.

...Transport-Canada has approved the Kenn Borek STC to remove it

That is correct. Clearly TC thought that the STC was worthy of approval.

...and the 400 series doesn't even have it...

That is also correct. Clearly the manufacturer of the Series 400 Twin Otter thought that the system was not necessary, and TC agreed. Note, however, that the Series 400 does have a system to indicate to the pilot when the propeller enters the ground fine range. If the propeller enters the ground fine range when the aircraft is on ground AND the pilot has commanded ground fine range (evidenced by twisting the power levers, which activates the switch above the power levers), the background behind the Np indication changes to a blue colour, as shown in the illustration below.

But, if the propeller enters the ground fine range when the aircraft is in flight, a red PROPELLER LOW PITCH Crew Alerting System (CAS) message appears, an aural warning "Propeller Low Pitch" is voiced, and the Master Caution annunciator illuminates. So, although the backup system itself has been deleted from the Series 400, annunciation of propeller entry to the ground fine range is retained - both for normal and abnormal circumstances.

Hope this answers your questions.


Series 400 Annunciation of Ground Fine Range (commanded)

V1... Ooops
2nd Dec 2010, 02:36
MachTwelve (what is your first name, anyway?):

The AFM abnormal I was actually referring to was 3.2.3 Inflight Propeller Reversal.
The procedure you refer to for 'inflight propeller reversal' (item 3.2.3 at revision 43 status) is totally unrelated to the beta backup system procedures. I realize that the causal factor for such a problem would, in theory, be something that should be caught (and prevented) by the beta backup system, but don't mix apples and oranges by trying to read too deep into the probable cause of the potential problem that this procedure addresses.

What the AFM says is (in summary) this: If a propeller enters the reverse range during flight and the aircraft does not have propeller blade latches installed, shut down the engine. If the aircraft does have propeller blade latches installed (these are normally only found on aircraft that are operated on floats for all or part of the year), then don't shut the engine down, just move the power lever to idle.

That procedure makes sense. For your information, I have reviewed engineering, certification, and flight test data that shows that with engine power at idle, a fully reversed (-15° blade angle) propeller actually creates less drag than the same propeller at +11° blade angle. This is because there is less flat plate drag presented at -15° than there is at +11°. So, the 'inflight propeller reversal' - a pretty remote possibility with the propeller control system installed on the Twin Otter - is not something that you should stay up late at night worrying about.

...can you comment on the option of twisting the power levers to dis-arm the system...

If you refer to the "Safety of Flight Supplement" number 4, dated 1 February 1981, which is found at the beginning of the Supplementary Operating Data (PSM 1-63-1), you will see that de Havilland explicitly forbids doing this.

For either one of the beta malfunctions (steady light or flashing light), take your time, go slowly, correctly identify the nature of the malfunction, correctly identify the right checklist to use and get agreement from the other pilot (if you are in a two-crew environment), then carry out the checklist actions as "read aloud and do", not from memory.

As I said earlier, if you were to freeze up in terror and do absolutely nothing in response to a steady beta light, the only consequence would be that you would get a sore leg from holding in all the rudder during the single-engine flight that would result. If you were to freeze up in terror and do absolutely nothing in response to a flashing beta light, the only consequence would be that when you finally brought the power back for landing, the aircraft would yaw back and forth a little bit (entirely controllably) during the flare.

However... if you act in haste and screw up and carry out the wrong procedure (e.g. complete the 'steady beta light' procedure when the problem is in fact a 'flashing beta light'), the consequences could be quite severe.

You have to keep in mind that the ONLY time that the propeller can go into a very fine pitch (meaning, less than +11°, with the reverse range from 0° to -15° being sort of an 'ultra fine pitch') is when the propeller is underspeeding relative to the propeller governor. This means that during the takeoff, climb, and cruise phases of flight, it is impossible to get a flashing beta light, because during all those phases of flight, because the large amount of power (torque) being applied to the propeller is keeping it under the control of the governor, which modulates the oil supply in order to maintain the propeller RPM selected by the pilot.

The only time that the 'flashing beta light' malfunction can appear is during the approach and/or landing phase of flight, when the power being applied to the propeller is reduced to such a low value that it is insufficient to keep the propeller turning at the RPM selected by the pilot. If the flashing beta light malfunction does appear, and the pilot adds power (one of the two items the checklist calls for the pilot to do), the addition of power will increase propeller RPM and hand control of the propeller back to the governor. The problem will disappear, at least, it will disappear until the power lever is next pulled back and the propeller is unable to maintain the speed selected using the PROP lever.

...don't really understand why it was mandated and thence effectively (via both the STC and the 400) the requirement was removed.It was mandated because the design of the propeller control system used on the Twin Otter - which provides direct control of propeller blade angle via the power lever during approach and landing - was considered new and novel when it was first introduced in the 1960s. Therefore, the regulators at the time asked for an independent electrical (rather than mechanical) backup system to prevent the propeller from entering the ground fine range during flight should the mechanical system somehow fail. This was prudent on their behalf and entirely understandable given the lack of flight experience with this new and novel propeller control system.

40 years later, we have the benefit of having learned from (literally) millions of hours of Twin Otter flight time that:

1) If the mechanical propeller control system fails, 99.9% of the time, the result is that the propeller feathers. That is the least hazardous thing that the propeller could do as a result of a failure, and no back-up system is needed if the propeller feathers.

2) Although the theory behind the beta backup system is sound, in practice, the system causes more problems (unwanted featherings as a result of the 'steady beta light' scenario) that it prevents. Therefore, regulators have approved STC modifications permitting removal of the backup system - although the INDICATION system (lights that show when the propeller is in the ground fine range) must remain.

Be aware, however, that during the 1980s, de Havilland introduced an optional modification to the design of the beta backup system (Mod 6/1831, S/B 6/478 at Rev A refers) that replaced the propeller mounted microswitches with a proximity switch, and this modification has virtually eliminated all 'steady beta light' malfunctions, except of course for those arising from a rigging problem, which would in any case be detected on the ground as soon as the engine was started up following the maintenance activity. This modification can be easily retrofitted to all legacy DHC-6 aircraft.

In the case of the Series 400, a very clear alert (text message and voice announcement) is made to the pilot if the propeller ever enters the ground fine range during flight. The actions that the Series 400 pilot then takes are identical to the actions set out for the "flashing beta light" for a Series 300 aircraft.


You may also want to review and compare the actions set out in the AFMs of single engine aircraft that use a similar engine and propeller control system (e.g. Cessna Caravan, Pilatus PC-6, Pilatus PC-12) and do not utilize a 'high idle' fuel control position such as the Beech products. Aircraft that utilize a 'high idle' position are, mechanically, sufficiently different that it is not appropriate to make a direct comparison with the Twin Otter.


V1... Ooops
2nd Dec 2010, 16:50
Hi Pete:

I would highly recommend that the operators of those aircraft consider fitting Mod 6/1831. That would address the root of the problem, and eliminate any perceived need by the pilots to follow non-standard procedures to cope with problems.

Below is an image of the "Safety of Flight" supplement that DH promulgated in 1980.


3rd Dec 2010, 10:11

Many thanks for the in-depth comments and remarks.

About twisting the power lever in flight, a good reminder not to do it is the March 2001 accident of F-OGES on approach to TFFJ. Prior to the accident the pilot reported that he used to use "beta" in flight in order to slow down during the [non-conventional] approach to this particular runway. Video footage (and audio) taken by a PAX during the flight was retrieved from the wreckage and supported the probable cause.

PROBABLE CAUSES: "The accident appears to result from the Captain's use of the propellers in the reverse beta range, to improve control of his track on short final. A strong thrust asymmetry at the moment when coming out of the reverse beta range would have caused the loss of yaw control, then roll control of the aircraft.

Accident description (http://aviation-safety.net/database/record.php?id=20010324-0)
BEA Accident Report (http://www.bea.aero/docspa/2001/f-es010324/pdf/f-es010324.pdf)

V1... Ooops
11th Dec 2010, 14:01
Hello Pete:

I don't think there is a reference in the Revision 52 (current version) AFM or the current version Supplementary Operating Data that addresses how long to wait to allow fuel to settle prior to using the dipstick.

The Series 300 AFM has recently been completely revised, and this new revision of the AFM (all pages are at Revision 53 status) will be distributed to operators who subscribe to the document revision service sometime within the next few months. I believe that the document is at the printers now, and I am going to GUESS that it will be distributed to subscribers sometime in the first quarter 2011.

I have attached an image below showing the remarks about the dipstick that are contained in this new (Revision 53) AFM.

A note to everyone: I cannot, and will not participate in any further discussion about the contents of the Revision 53 AFM until it is distributed. To do so would just cause chaos, I am sure you can all appreciate that.

Here's the blurb that addresses use of the dipstick. Note that it is by no means obligatory to use the dipstick.


22nd Dec 2010, 17:18
In the Arctic we started in fine pitch on all models 100, 200 & 300. It started oil flowing to the prop and thus lubricating the shaft. We had some initial problems with shaft bearings due to the different coefficients of expansion at those temps. At those temps we would feather at least 4 or 5 times after the start to keep the oil flowing. And this was after about 30 minutes with Herman in each intake (engine covers on).

11th Oct 2012, 18:51
SOP in Canada's Arctic was the F/O would hold the prop until you had oil temp. this was to avoid blowing the prop seals with the thick oil.

12th Oct 2012, 10:12
Never held the prop once for this reason and never heard of anyone else doing it either. Tried to hold a prop once during a start and I remember that it took quite a bit of force. Mind you we always plugged in our engines.

Besides, why not let your oil warm up while in feather. Isn't it actual prop blade blade angle change that will damage seals?