Go Back  PPRuNe Forums > Flight Deck Forums > Tech Log
Reload this Page >

DHC-6 Questions:

Tech Log The very best in practical technical discussion on the web

DHC-6 Questions:

Old 26th May 2008, 16:06
  #1 (permalink)  
Thread Starter
 
Join Date: Apr 2003
Location: Germany
Age: 71
Posts: 1,560
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.
chuks is offline  
Old 26th May 2008, 16:37
  #2 (permalink)  
Sir Osis of the river
Guest
 
Posts: n/a
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........
 
Old 26th May 2008, 16:47
  #3 (permalink)  
 
Join Date: Mar 2008
Location: SA
Age: 45
Posts: 67
All makes sense Sir Oasis, and you right a strong x-wind in a Twott is not for sissies!
flux is offline  
Old 26th May 2008, 17:00
  #4 (permalink)  
 
Join Date: Nov 2003
Location: Planet Tharg
Posts: 2,471
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.
Solid Rust Twotter is offline  
Old 26th May 2008, 18:30
  #5 (permalink)  
 
Join Date: Feb 2005
Location: Canada / Switzerland
Posts: 507
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
V1... Ooops is offline  
Old 26th May 2008, 18:45
  #6 (permalink)  
 
Join Date: Feb 2005
Location: Canada / Switzerland
Posts: 507
Originally Posted by Solid Rust Twotter
...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.

Michael
V1... Ooops is offline  
Old 27th May 2008, 03:14
  #7 (permalink)  
Sir Osis of the river
Guest
 
Posts: n/a
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
 
Old 27th May 2008, 11:54
  #8 (permalink)  
 
Join Date: Apr 1999
Location: The land of chocolate and cuckoo clocks!
Posts: 136
Talking

Hola V1... When I saw this post I just knew that I would find you here!
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 knowing pilots...one can say sometimes
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
Option A was a long runway so an eventual landing/arrival was made with Flaps 10 and a strange angle on the horizon
Option B cannot be talked about here otherwise V1 will be visiting me sooner than I expect him
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
Happy flying in this unique member of the dehavilland family
Rat Catcher is online now  
Old 27th May 2008, 22:52
  #9 (permalink)  
 
Join Date: Jul 2000
Location: London
Posts: 200
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!
smallfry is offline  
Old 28th May 2008, 00:52
  #10 (permalink)  
 
Join Date: Feb 2005
Location: Canada / Switzerland
Posts: 507
...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
V1... Ooops is offline  
Old 28th May 2008, 07:04
  #11 (permalink)  
 
Join Date: Jul 2000
Location: London
Posts: 200
Dont worry V1. private joke between me and Rat Catcher. Sadly my 'bush' flying days are over. All by the book now!
smallfry is offline  
Old 28th May 2008, 07:20
  #12 (permalink)  
 
Join Date: Feb 2005
Location: Canada / Switzerland
Posts: 507
Ah, OK, missed that nuance, sorry.
V1... Ooops is offline  
Old 28th May 2008, 08:01
  #13 (permalink)  
 
Join Date: Apr 1999
Location: The land of chocolate and cuckoo clocks!
Posts: 136
Snoop

Good to see you on Small Fry! VI is an old pal
Food for thought about your "bush" flying..perhaps when "George" retires, you'll get to fly him!
Rat Catcher is online now  
Old 28th May 2008, 08:57
  #14 (permalink)  
 
Join Date: Jul 2000
Location: London
Posts: 200
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!
smallfry is offline  
Old 28th May 2008, 16:30
  #15 (permalink)  
Thread Starter
 
Join Date: Apr 2003
Location: Germany
Age: 71
Posts: 1,560
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?
chuks is offline  
Old 30th May 2008, 05:09
  #16 (permalink)  
 
Join Date: Apr 1999
Location: The land of chocolate and cuckoo clocks!
Posts: 136
Angel

Small Fry...I am wounded I always behave Perhaps we should not get into the many definitions of that word though
See you soon my friend
Rat Catcher is online now  
Old 25th Apr 2010, 10:23
  #17 (permalink)  
 
Join Date: May 2004
Location: Australia
Posts: 43
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?
Lando Calrissian is offline  
Old 25th Apr 2010, 11:02
  #18 (permalink)  
 
Join Date: Mar 2010
Location: Mayberry
Posts: 135
Twotter starts........

You start in feather and do not exceed 10 lbs of tq in feather. per the book.
Spadhampton is offline  
Old 25th Apr 2010, 19:39
  #19 (permalink)  
 
Join Date: Feb 2005
Location: Canada / Switzerland
Posts: 507
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 (10C) 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... )
V1... Ooops is offline  
Old 25th Apr 2010, 19:44
  #20 (permalink)  
 
Join Date: Oct 2008
Location: Mexico
Age: 18
Posts: 12
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.
contrabando is offline  

Thread Tools
Search this Thread

Contact Us Archive Advertising Cookie Policy Privacy Statement Terms of Service

Copyright 2018 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.