Twin Otter Beta Use in Flight

[NWONT]

Flight-Safety Checklist

[Cliff Jumper]

This is one of those times that I know more than I'm willing to post on a public forum, so take my word for it, no matter how good you think you are, you can still loose control of a DHC airplane attempting flight behind the gate.

Or to put another way if you select behind the gate in flight you are not a good pilot you are deliberately operating outside the C of A of the airplane and you are an unprofessional pilot.

Now, please don't misunderstand me. I'm not promoting placing the the throttles below flight idle at any time during flight.

What I'm suggesting is that this information is confusing, and could easily be misinterpreted by someone. If the truth is that beta is used on every approach/landing while airborne, then we should immediately change all guidance, manuals, documents, pilot training, accident reports etc to say so.

That is to say, "entering beta in flight is ok, however moving the throttles below the flight idle position, is not". And of course, change the definitions of 'beta range' to not discuss thrust lever position, but rather to discuss propeller blade angles.

Wasn't it simpler and safer when we just called 'beta range' the throttle position below flight idle?

I can just see my F/O now... "props full forward, entering beta" ... I'll lose my f'in marbles, but apparently he's completely correct.

[NWONT]

Pull the breaker....twist the grip momentarily for a steady beta light....both do the same thing.....disable the beta-backup system.....right?

[PilotDAR]

What I'm suggesting is that this information is confusing, and could easily be misinterpreted by someone.

Ah, not if they've received competent training. Just under 1000 Twin Otters have entered service, the oldest about 52 years ago. Why would this be a concern now? Sadly, there will always be cowboy pilots, who you can't tell anyway. If someone has given them a Twin Otter to fly, well, mistake one. Otherwise, I've never heard of a properly trained pilot not understanding what is permitted or prohibited in flight.

[GyvAir]

Paneuropean, I know your knowledge of the Twin Otter is far beyond most of us "BUT" see the attachment. It clearly says that flight-safety teaches to feather the prop for a steady beat light.....and you mixed two of my posts to make a false statement. I didn't say the MNR taught to use beta in flight..I said we use the sim to teach our off strip methods which include lifting off soft strips at less than the accepted speeds and how to maintain control if an engine fails.

I see where you say Paneuropean mixed two parts of your earlier post together (and why he would have seen them as related), but I'm still trying to figure what you're saying here, on its own:

"I've been retired for some time but one way they teach is if you have a steady beta light you are to power back, feather the prop pull the breaker....then bring it back. So much easier to just twist the grip if you need a few extra seconds to find the breaker. They said it was because some fool did something wrong so they had to make it fool proof!!!....OK, roger on that!!!"

What does twisting the grip for a few seconds do for you in that scenario?

To be clear, I'm not trying to be a smartass, I'm just not getting the mechanics of what you're saying there.

[Cliff Jumper]

What I'm suggesting is that this information is confusing, and could easily be misinterpreted by someone.

Ah, not if they've received competent training. Just under 1000 Twin Otters have entered service, the oldest about 52 years ago. Why would this be a concern now? Sadly, there will always be cowboy pilots, who you can't tell anyway. If someone has given them a Twin Otter to fly, well, mistake one. Otherwise, I've never heard of a properly trained pilot not understanding what is permitted or prohibited in flight.

Well, actually, I think we've seen more 'inadvertant' beta accidents in DHC6's than in other turboprops..... or am I mistaken?

Perhaps this is what is leading them to think it's ok. As soon as somewhere hears 'approach beta' is perfectly ok on a DHC6, and then clearly understands that 'beta mode' is below flight idle... put two and two together.

I'm not trying to suggest that this issue is 'the end of the world', it's seems however to be a fundamental misunderstanding, which I'm willing to be included in, although I believe I was 'properly trained'. I would never retard power levers beyond flight idle in flight. I advance props to full during approach.

I did not really consider the range between 11 and 17 degrees to be 'beta' as I believed that cumulative forward thrust is still being produced. I know the beta-reverse valve is controlling pitch during this time, but I don't believe that simply because this valve is operating that this meets the definition of 'beta range'. To me, beta range of the propeller means any time the 'average' chord of the propeller is at an angle of attack to the relative airflow of 0 deg or less. To me, this is universal to all constant speed propellers. Alpha forward, beta back.

But I acknowledge that I'm a simple man.

[NWONT]

Gyvair. If you are flying single pilot.....at night...and a steady beta light come on then the valve has closed, shutting off oil flow. The beta back-up system has malfunctioned so you want to disable it. If you need a second or two to find the breaker, you can twist the grip, which will disable it, all is good now...and locate the breaker and pull it. Or you can power back, feather the prop, then find it, pull it, then unfeather and re-adjust your engine. If you have mistaken a flashing light for a steady light...then twist the grip.....well.. shit's gonna get real!!! The first thing you will notice is your props slowly go out of sync....you have time to observe and make the proper actions. I had this discussion at Flight-safety.....and they don't agree with my method and I have used it when I had a steady light and it works fine. Paneuro says Flight-Safety has never taught the feathered engine method. Well it's right in their check list....and NO!! I don't want an extended argument with Paneuro, he is a Twin Otter Guru and I say that respectfully.

[NWONT]

Paneuro, you chopped up my post, fitted the words together to suit your purposed and totally misrepresented what I said. Don't do that again!!....or I'm going to remove that "respectfully" comment I made!!!

[cncpc]

I have heard of people using "beta" decent after dropping sky divers - never thought it was smart but some had the big cahonies -- the porter on the other hand beta was used a lot for approaches and decent.

it is used with the Porter to slow it to the same speed as the freefall jumpers. I'm not sure why that is necessary, photos maybe.

[PanEuropean]

I'm quite surprised to see that the FlightSafety checklist image posted at the top of this page, which has a revision date of 2006 (and therefore is most likely not the current version), advises the pilot to feather the propeller.

The most recent version of the AFM/POH that I have a copy of (I retired in 2013, this revision was current then) does not mention feathering the propeller in the event of a steady beta light. No previous version of the AFM ever mentioned feathering the propeller in the event of a steady beta light. I do not know if this procedure has been revised since I retired - perhaps someone with a current copy of the AFM could check and let us know.

Below is an image of the AFM procedure, at revision 53.

[NWONT]

Check what you posted!! You said FS had NEVER used this procedure!!! This checklist is dated 2010. That was pretty arrogant of you. You also spliced my words from two posts to come up with bullshit I never said.....NOT NICE!!! I recall a few years back you got brought down a peg or two about ferrying Twin Otters. Don't let it bend you out of shape......I think I know everything too!!!

[NWONT]

I just noticed...the Viking check list is dated 2010 also!!! Like I said, Flight-safety trains with lawyers in mind. Twin Otters can do a helluva lot more than Flight-safety says they can!!!

[PanEuropean]

I'm still trying to figure out why they designed to propeller on the Twin Otter to not be able to go into reverse unless the prop levers are above 91% (full forward is 96%) unlike the King Air which doesn't seem to have this restriction.

Pelmet:

I can't comment on the King Air, I have no knowledge of that aircraft. But, I can explain why the propeller levers have to be full forward (set to 96%) before you can move the power levers aft of the idle stop (into the ground range area) to use reverse thrust. It's kind of a lengthy explanation, because I have to provide some background information before getting to the point and providing the answer.

A propeller on a Twin Otter can be controlled either by the constant speed portion of the governor, or by the beta-reverse valve. Both of these sub-systems use the same methodology to control the propeller (they supply oil under pressure to move the propeller towards a finer blade angle, thus opposing the action of the internal spring and external counterweights which are always trying to drive the propeller towards feather, feather being a coarser blade angle). Although they use the same methodology, they function to achieve different objectives. The objective of the constant speed portion of the governor (the CSU) is to maintain a propeller RPM selected by the pilot using the propeller lever, and the CSU maintains a set RPM by varying blade angle as required to achieve the set RPM. If RPM is too slow (underspeeding), the CSU admits more oil to the propeller to fine out the blade and enable it to rotate faster (due to less resistance to rotation). If RPM is too fast (overspeeding), the CSU reduces the oil supply, and the propeller moves to a coarser blade angle, which slows it down (due to greater resistance to rotation).

The objective of the beta-reverse valve is to maintain a set blade angle, without any concern at all for whatever the RPM may be. When the propeller system is rigged by the technician, the technician sets the idle blade angle at +11°. As long as the power levers are not moved aft of the idle stop, into the ground operations range, the beta-reverse valve will ensure that the blade angle does not decrease below +11°. When the pilot twists the power levers and pulls them aft, into the ground operations range, a linkage connected to the power lever pulls on a lever attached to the beta-reverse valve and opens the beta-reverse valve, admitting more oil into the propeller. The propeller blade angle decreases. There is a follow-up linkage connected to the a feedback ring on the propeller dome that moves the beta-reverse valve back towards a closed position, thus regaining an equilibrium at the position the pilot has selected when he or she pulled the power levers aft. If the pilot pulls the power levers further aft, the same process repeats: beta-reverse valve opens a bit, oil flows into the prop, prop dome moves outwards, blades move to a finer blade angle, feedback ring moves beta-reverse valve back towards closed, equilibrium achieved once again, this time at a finer blade angle.

Now, before we go further, take careful note that in each of the above scenarios, both the CSU and the beta-reverse valve use the same method of moving the propeller towards finer blade angles: they admit more oil into the propeller dome. And, they use the same methodology to move the propeller towards coarser blade angles: they restrict or even cut off the flow of oil going to the propeller.

From the above, we can extract two key rules necessary to understand propeller behaviour:
1) It takes A LOT of oil to make the propeller move to a finer blade angle (full reverse being the 'most fine' blade angle), and;
2) If you cut off (or substantially reduce) the oil supply, the propeller will move to a coarser blade angle (feather being the extreme end of a coarse blade angle).

Keep those rules in mind, they will come in handy later on in this explanation.

If the CSU can get the propeller to achieve the pilot's selected RPM (as set with the propeller lever), then the beta-reverse valve stays wide open and does not interfere with or control oil flow to the propeller. But, if the CSU cannot get the propeller up to the set speed, the propeller will keep moving towards a finer and finer blade angle as the CSU continues to supply oil to the propeller. Eventually, the beta-reverse valve will step in and say, in effect, "that's fine enough, I'm not letting you move that propeller to a finer blade angle than +11°", and the beta-reverse valve will start moving towards the closed position and reduce oil supply to the propeller in order to limit blade angle.

Based on what has been said so far, you can extract two conclusions: 1) The CSU always gets "the first opportunity", so to speak, to control the propeller, and; 2) The propeller has to be in an underspeeding condition (in the opinion of the CSU) before the beta-reverse valve will operate.

The reason that the pilot is asked to push the propeller levers forward at low power settings (by way of the 'RESET PROPS' light illuminating) is to force the propeller into an underspeed condition, thus handing control of propeller blade angle over to the beta-reverse valve. Remember, as long as the CSU can achieve selected propeller RPM, the beta-reverse valve will not operate... it will remain wide open, allowing the CSU to control blade angle.

Now, getting to your question (I'm paraphrasing it) "Why is it necessary to move the propeller levers fully forward to release the mechanical interlock before you can twist the grips and move the power levers back into the ground operations range?"

Easy to answer. Just consider the following scenario:

1) Pilot is landing on a very, very, short strip.
2) At the end of that very short strip is the town cesspool, and it is full.
3) Pilot obviously wants to stop before running off the end of the strip.
4) But this particular (imaginary) Twin Otter has no mechanical interlock installed on it, so, it is possible to pull the power levers back into the ground range when the props are set to 75% Np.

The pilot flies a perfect approach, exactly on speed, touches down on the first 6 inches of the strip, and promptly twists the grips and hauls the power levers back into row 3 of the cabin. The propellers move to full reverse, and the engine starts to roar as Ng rises. As Ng rises, the propellers start turning faster and faster. As soon as the propellers reach 75% Np (which is what the prop levers are set at), what is the CSU portion of the propeller governor - which up to this moment, has been in an underspeeding condition - going to do? Guess what, it's going to start to govern Np at 75%. And how does it do that? By reducing the oil supply to the propeller. And what happens when you reduce oil supply to the propeller? See Rule 2 up at the beginning of this explanation.

So, here's our pilot, engines howling, power levers all the way back, coming ever closer to the cesspool at the end of the strip, and suddenly the CSU decides to substantially reduce the oil supply to the propeller (or even dump oil from the propeller) in order to limit RPM to 75%. The prop will respond by rapidly moving to a much coarser blade angle, and as soon as the prop coarsens up above 0°blade angle, it will develop forward thrust, even though the power levers are all the way aft. The pilot will wind up in deep s___, off the far end of the runway.

So, as you have probably figured out by now, the reason for the mechanical interlock is to make sure that the props are always set at the MAX RPM (96% Np) setting before the pilot can move the power levers aft of the idle stop, this to prevent the CSU from suddenly deciding it is going to "govern" the props when the pilot is commanding reverse thrust.

Hope this explains things satisfactorily.

Michael

PS: The 91% Np limitation you mentioned is a different matter. It is a restriction imposed on propeller speed when reverse is selected, by reducing fuel supply to the engine. The reason is the same - we don't ever want the CSU portion of the prop governor to become active when heavy reverse is selected. The selected Np in reverse is 96% (the mechanical interlock ensures this is so). To prevent Np from reaching 96% when reverse is selected, a totally different mechanism in the governor assembly reduces fuel supply to the engine as soon as Np reaches 91% when the power levers are aft of idle.

[AuxBatOn]

I just noticed...the Viking check list is dated 2010 also!!! Like I said, Flight-safety trains with lawyers in mind. Twin Otters can do a helluva lot more than Flight-safety says they can!!!

They teach to the flight-tested, cleared flight enveloppe. Doing anything else would be wreckless.

[PanEuropean]

This checklist is dated 2010.

Uh, no, the date I see at the bottom of the photo of the checklist you posted is September 2006.

I just noticed...the Viking check list is dated 2010 also!!!

Yup, that's correct.

[PilotDAR]

Flight-safety trains with lawyers in mind. Twin Otters can do a helluva lot more than Flight-safety says they can!!!

Ah, it is true! However, Flight Safety, as well as every other professional training organization I know of, trains the safe and legally complaint operation of aircraft. That's what their clients expect of them, 'cause that's what will keep the flight safe,, and the lawyers away. Every plane can do more than it says in the limitation section, 'doesn't mean we go and do it!

If there is a genuine operational need for a limitation expansion, you can apply for an STC, or a flight permit. Maybe TC will agree with you (overweight ferry is an example). In the mean time, appreciate that the aircraft was certified with limitations to give the average well trained pilot room to breath, fly the plane properly, and get home safely at the end of the day! It is surprising and disappointing that a few pilots seem to place so little value in the flight manual, and its limitations. One of the things I've learned during flight testing is that once you're outside limitations, things can become exponentially different with only a small exceedance - things going bad is not always linear. And then you add a system failure or other emergency on top of what you already should not be doing, and there's no getting back.

This thread has been completed as a wealth of knowledge about beta system function in a Twin Otter (thanks PanEuro). If you want a thread about "why should I follow the rules?" maybe that should be a new thread with a broader scope!

[NWONT]

The date on the front of the checklist says, Revision 3.2 November 2010....so obviously that page hasn't been amended to that point. Going for a margarita so won't be in this discussion for a week

[golden hawk]

Flight-safety trains with lawyers in mind. Twin Otters can do a helluva lot more than Flight-safety says they can!!!

Ah, it is true! However, Flight Safety, as well as every other professional training organization I know of, trains the safe and legally complaint operation of aircraft. That's what their clients expect of them, 'cause that's what will keep the flight safe,, and the lawyers away. Every plane can do more than it says in the limitation section, 'doesn't mean we go and do it!

If there is a genuine operational need for a limitation expansion, you can apply for an STC, or a flight permit. Maybe TC will agree with you (overweight ferry is an example). In the mean time, appreciate that the aircraft was certified with limitations to give the average well trained pilot room to breath, fly the plane properly, and get home safely at the end of the day! It is surprising and disappointing that a few pilots seem to place so little value in the flight manual, and its limitations. One of the things I've learned during flight testing is that once you're outside limitations, things can become exponentially different with only a small exceedance - things going bad is not always linear. And then you add a system failure or other emergency on top of what you already should not be doing, and there's no getting back.

This thread has been completed as a wealth of knowledge about beta system function in a Twin Otter (thanks PanEuro). If you want a thread about "why should I follow the rules?" maybe that should be a new thread with a broader scope!

Very well said. If lawyers are involved, it's because of negligence, such as knowingly operating outside of the AFM procedures and limitations.

One of the best threads I've seen on AvCanada. Many experienced pilots posting valuable information.

[Edo]

The reason that the pilot is asked to push the propeller levers forward at low power settings (by way of the 'RESET PROPS' light illuminating) is to force the propeller into an underspeed condition, thus handing control of propeller blade angle over to the beta-reverse valve.

Older Beechcraft products (maybe new ones too) have a "reverse not ready" light that comes on with lower power settings when the props have not been advanced to the forward stop. - seems more "pilot proof" than "reset Props"

Tom Clements has a great article about the F90 similar scenario to the "cesspool" above.

Gusty day =ref +10 maybe +20 poor speed control on final, you cross the threshold fast. Haul the Power lever over the gate - due to excess (ground) speed and ram airflow the prop is not in an under-speed condition = NO reverse

Apparently more than one F90 pilot has hit the rubarb, and the stated reason was REVERSE DIDN'T WORK

Nope...... excess speed prevented the prop from reaching an under-speed condition (regardless of prop lever position)