Twin Otter Beta Use in Flight

[PanEuropean]

Hello Cliff Jumper:

Sorry, I made a dumb typing error in the first post, the second statement of mine that you quoted should have read "propeller operating at a speed less than what is selected with the PROPELLER lever" . Very sorry about that, and thanks for catching that error - I will edit the original post to correct the typo, if the forum software permits me to do so.

And, yes, if you are sitting at the end of the taxiway waiting for take-off clearance, the propellers are in beta range, because the actual propeller RPM is less than what you have selected with the propeller levers.

I will leave it to fellow forum member Pilot DAR to elaborate further - his engineering background is far more scholarly than mine.

---------

Also, I need to make a correction to what I wrote above (I've been retired for several years now, and it's been over 10 years since I used the sim at FlightSafety) - we programmed the sim to freeze anytime the power lever grips were twisted if the aircraft was not on the ground (which would include the steady beta light fault, if it was presented in flight). This kind of 'lockout programming' is very common in simulation, to prevent fools from trying to experiment in a simulator with practices that the simulator has not been certified for. In other words, during simulator development, no data was ever collected with the power levers aft of the idle stop in flight... therefore, any simulation of this scenario would be speculative, rather than based on fact. For that reason, it's locked out. The same concept explains why an Airbus 380 simulator (or any other large transport aircraft simulator) will freeze if you try to roll it past 66 degrees of bank.

[Cliff Jumper]

Interesting PanEuropean,

So, basically you are saying that any time the governor is controlling the propeller rpm it's 'alpha', and any other time is 'beta'?

This seems to differ significantly from the many text books written on the subject.

I wonder why we would refer beta to it in this fashion, it seems a needless term. Now every constant speed piston has beta, and operates in the beta range alot. It seems we need a term to describe 'forward thrust' and 'not forward thrust', not a term to describe whether or not the governor is doing its job.

[PanEuropean]

So, basically you are saying that any time the governor is controlling the propeller rpm it's 'alpha', and any other time is 'beta'?

That is my understanding, and that is what I have written.

As I mentioned earlier, I'm going to leave it to fellow forum member Pilot DAR to make the final call (& full explanation) about this, because he is much better qualified than I to do so.

Michael

[golden hawk]

not many who understand the term 'beta'.....except GH

Or perhaps a chance that you don't.

It's a long debated topic, but my stance is that there are different meanings depending on what you're talking about.

Beta range of the power lever is any position below flight idle.

Beta range of the propeller (in my opinion) is any pitch below the low pitch stops.

Thank you to PanEuropean for providing the facts on this.

BETA.jpg (274.68 KiB) Viewed 2180 times

[Cliff Jumper]

Sorry PanEuro if my post came off suggesting that you're wrong, that's not what I intended. I'm genuinely confused... thought I had it clear, but obviously you know what you're doing, and your understanding is different than mine.

GH, thanks for the quote. That would be consistent with what PanEuro is saying... perhaps he even wrote it.

I don't have scanned copies of the other texts handy, but here's a few webclips, which suggest otherwise. (why would entering beta in flight be dangerous if beta included everything up to the governing range?)

https://www.skybrary.aero/index.php/Beta_Range

https://www.faa.gov/regulations_policie ... h_ch14.pdf

http://wwwapps3.tc.gc.ca/Saf-Sec-Sur/2/ ... 15R2-E.pdf

https://www.atsb.gov.au/publications/in ... 159/si-01/

https://www.faa.gov/regulations_policie ... 112699.pdf

[PanEuropean]

Sorry PanEuro if my post came off suggesting that you're wrong, that's not what I intended. I'm genuinely confused... thought I had it clear, but obviously you know what you're doing, and your understanding is different than mine.

Hey, no apologies needed - I was wrong! I made a typo (I referred to the power lever instead of the propeller lever), and you caught it. That means I owe you a coffee, at the very least. As soon as I saw your post, I went back and corrected the typo in my original post.

The photograph above (that Golden Hawk posted) of the portion of the page from the FlightSafety training manual that explains beta range should, I think, fully explain things. It is also my writing, but from about 23 years ago.

Michael

[PanEuropean]

Cliff Jumper:

Below is an illustration that I think might clear up any confusion. It shows the operating range of a Series 300 and Series 400 Twin Otter propeller. I've annotated it a bit to highlight the difference between "beta range" and "ground operations range".

When the propeller governor is controlling propeller speed, the propeller is in "constant speed range". The pilot can easily determine this by looking at the Np gauge. If the propeller RPM you see on the Np gauge matches the propeller speed you have selected with the PROP levers, you are in constant speed range.

The propeller blade angle will vary in constant speed range, but it will always be greater than +17°. Exactly what the blade angle will be depends on atmospheric conditions, how much power is being delivered to the propeller, and the speed of the aircraft. I have been told that during typical cruise flight, the blade angle is somewhere in the +20° to +35° area, but I don't know how accurate that statement is. I do remember from my time at FlightSafety back in the 1990s that we could look at a debug screen on the simulator and see what propeller blade angle was calculated to be at any given moment.

In constant speed range, the governor adjusts blade angle to maintain the selected RPM. The beta-reverse valve on the front of the engine is wide open all the time during constant speed range.

When propeller blade angle decreases below +17°, a linkage in the front area of the engine connected to the propeller begins to move and partially close the beta-reverse valve. The propeller control system is rigged so that an equilibrium is reached at +11° (the flight idle position of the blade when at low speed and low power). The propeller blade angle will not decrease below +11° unless the pilot "twists & pulls" the power levers. The blade angle range between +17°and +11° is, of course, available for use in flight. This particular range is referred to as 'approach beta'. It is a sub-set of 'Beta Range'.

In beta range (including approach beta), the beta-reverse valve directly controls blade angle, without any concern for what the RPM may be. In beta range, the propeller governor is always in an underspeed condition, and therefore has no control over blade angle or RPM.

It is not possible to enter beta range unless the propeller is underspeeding relative to the speed the pilot has selected with the propeller levers. This is why the AFM/POH tells the pilot to push the propeller levers forward when setting up on final approach. This is also why the Twin Otter has a caution light (or CAS message, on the 400 Series) that says "Reset Props". The "Reset Props" caution light or CAS message says to the pilot: "Hey, you, I can tell from your power lever setting that you are (or soon will be) going slowly enough to suggest that you are landing, so, push those friggin' propeller levers forward, that will put the propeller into an underspeed condition and give propeller blade angle control over to the beta-reverse valve. And, as every well-educated pilot knows, the beta-reverse valve is connected on one side to the propeller, and on the other side to the power lever. This means that if the pilot forces the propellers into an underspeed condition by pushing the prop levers forward when the "Reset Props" light illuminates, the pilot will then have direct control over propeller blade angle via the power lever. The effect is like shifting your Twin Otter into four wheel drive - you have immediate and direct control over both fuel supply to the engine (Ng) and propeller blade angle in one hand, that being the hand on the power levers.

Poorly educated pilots don't comprehend this. They just don't like the slight increase in cabin noise that happens when the propeller speed increases as a result of pushing the prop levers forward when the "Reset Props" light comes on. So, they keep the prop levers back at 75%, where they have been since the after-takeoff checklist was completed, and fly their approach with the governor controlling the prop, keeping it at 75% Np. By doing this, they completely disable and over-ride a very carefully engineered system that was designed to give them direct and immediate control of both blade angle and fuel scheduling when on final approach. They have no more control over blade angle, speed, and descent profile than a pilot of a PA-28.

Michael

[PilotDAR]

A lot of very useful information has been presented. It's difficult in the forum format to make this concept entirely clear, as you really need to be trained, then try it to understand the system. It may lend some clarity to add that the blade angle values are at the stated 30 inch station. The blades have quite a twist, so that is the blade angle only at that station. Thus, in beta range (not behind the gate yet though) the outer part of the propeller disc may be at a reverse blade angle, a middle portion at true zero thrust, and the inner portion still in forward thrust. The net effect of all of that may be zero thrust, or a little forward, which is why certification has established flight is acceptable in that configuration. And, the engines are hardly developing any power there anyway. Once you select behind the gate, you're beginning to get to more effective reverse because of blade angle, and starting to develop more power too.

All of the foregoing (being a discussion about beta) is in contrast to the excellent MT reversing propeller available for piston engines. That is reverse, without being beta. In that installation, the throttle never controls blade angle, and once selected by electric switch, the blades move to one [reverse] angle only, though the inner part of the blade will still produce forward thrust. They do not go more or less reverse. This helps keep engine temperatures somewhat reasonable, while you're actually otherwise going backward through the air. MT has designed in brilliant interlocks to absolutely prevent the blades moving the reverse angle in flight.

[pelmet]

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.

It seemed that the prop levers being moved forward on touchdown(o perhaps not having been moved all the way forward, or having slipped aft) could lead to the power levers hanging up(unable to be moved further back into reverse).

Based on my own experience of not being able to get reverse a couple of times(fortunately on long strips), I suspect that this was the initiating factor in a deadly accident involving a go-around after being unable to get reverse on a short strip.

[Cliff Jumper]

Well, I guess I am the only one who's confused on this topic. Apparently I'm dumber than I thought.

Could you clear these up for me? I'll put it in bullets to make it easy to respond (thanks bye the way).

1. You consider 'beta' to be defined as 'any range of propeller operation when the governor is not controlling the blade angle'
2. Is this a universal description or DHC6 specific?
3. The blade angles between 17 and 11 (when measure at a specific station) are included in 'beta'.
4. Is this determined to be 'beta' because
a) it meet the definition of prop speed below selected, or
b) because it's a negative angle to the relative airflow, or
c) because the blade angle is being controlled by the beta-reverse valve.
5. If it's a neg/airflow angle, then isn't this actually a form of reverse? In flight???? If not, why are we using this term. Is this what alpha/beta means in an engineering context?
6. I've never heard the term 'approach beta' other than in the diagram you included, and in the 2001 St.Baarts accident report. Is this unique to the DHC6?
7. You've made it clear that 'beta' is not allowed to be used in flight. In fact, so has everyone. However you've then suggested that it's used on every single landing.
8. It would appear the Nepal pilot who said they are 'approved for beta use in flight' are absolutely correct. On every approach, they select props full forward, and once the airspeed reaches a certain value, and the beta-valve starts controlling the pitch between 11 and 17 degrees, they are in beta.

Thanks again for the insight.

[PilotDAR]

Think of the "beta line" as being the fine pitch stop for the prop. If you want the most forward thrust you'll select fine pitch, and if governing is required, the governor will do it. If you're not requiring lots of forward thrust (on approach, for example), your interests in a shorter, steeper approach may be served with the blade angle set more fine than fine pitch, where perhaps a portion of the blade is not producing forward thrust anymore. The setting for that will be airplane type specific. In my experience flying Twin Otters and Caravans, the effect is noticeable. My King Air flying is very limited, and my Cheyenne flying so long ago, I simply don't remember. In all such cases, the governor has stopped governing, as the blades are already at the fine pitch angle - either because the pilot has selected props full fine, or the reduced power allowed them to move to full fine anyway (springs, I think). But none of that has yet involved the pilot moving the power levers behind the gate.

Beta is happening when at idle or "less" selected power lever position, the blade angle is changing because of a power lever position change (cams/levers/valves). In early beta, some part of the blade may be at a negative blade angle, but more certainly the blade angle is more fine than required for forward thrust. If there is a small portion of the prop disc developing reverse thrust in beta but not behind the gate, that's the design of the aircraft, and that operation has been approved. That's why the gate is there, so the pilot knows where the line is between approved in flight and not approved in flight power lever (=blade angle) position.

"Approach beta" seems to be a troublesome term. It could lead to a misunderstanding that there is a grey zone around the use of the beta gate, there is not. FOllowing the flight manual, if you have a propeller powered aircraft in the air, and the power levers have been selected behind the gate, you're doing it wrong. So, I've been hoping to make it clear that selecting power lever position behind the beta gate in flight is not allowed. What the powerplant is doing with the power lever position ahead of the beta gate in flight is none of your concern, other than you are to fly the aircraft safely within the skills of your training. In reference to the words attributed to the "Nepal pilots", I suggest that's playing with words to subvert the meaning. The fact that something is prohibited does not mean it is not done. I have seen videos in which I know for certain that the aircraft is well behind the beta gate. When an empty jumper Twin Otter is pointing straight down, and not accelerating, you know what's happening. At umpteen thousand feet, the experienced jump pilot may have a plan for what will certainly be reduced controlability of the aircraft. On short final, that reduced controlability could result in a crash.

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.

[Cat Driver]

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.

[NWONT]

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.

[NWONT]

?

[NWONT]

Flight-Safety Checklist

[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.