Twin Otter Beta Use in Flight

[Victory]

In the related videos there is one taking off and landing on turf in what looks like just a few hundred feet. This aircraft obviously has a very low stall speed. https://www.youtube.com/watch?v=ZP-9mWqFh3c

[bald seagull]

A lot of PC-12 discussion in a thread about Twin Otters.....

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

[Cliff Jumper]

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.

[goingnowherefast]

My Twin Otter Flight Safety book defines Beta range slightly differently than my King Air book. I have no idea how Pilatus defines beta, and I don't care as this is a twin otter thread.

[SuperchargedRS]

My Twin Otter Flight Safety book defines Beta range slightly differently than my King Air book. I have no idea how Pilatus defines beta, and I don't care as this is a twin otter thread.

Forest through the trees much?

[PilotDAR]

The key to "beta use" lies in what the pilot is commanding the propeller governor to do. The governor may be governing when the prop levers are pulled back to less than maximum RPM (depending upon the power selected with the power levers). If the governor is governing, the prop is not in beta range. If the prop levers are fully forward, and the power levers back, the blade angle may be moving into beta, or at that phase, disking. When disking, the engines really are not developing much power, so the effect is minor = permitted in flight, but the pilot has not passed the gates with the power levers.

"Using beta" implies that the pilot has passed the power lever gates, and moved the power levers further back, and now two things will be happening: The blade angle will be moving more toward reverse thrust, and the engine power will be increasing. The prop levers remain set to fully fine, even though the blade angle may be moving toward coarse in the reverse direction (governor is not governing), so the power levers are controlling both blade angle and engine power in unison. The pilot will have no doubt that they have made this selection.

I'm not aware of any propeller powered aircraft permitted the use of reverse thrust (behind the gate) in flight. I do recall that the DC-8-63 was permitted reverse on the inboard two engines in flight.

[PanEuropean]

I recently met a pilot who used to fly in Nepal...He said that they had approval to use Beta in flight.

He is incorrect. The type certificate holder (either DH in the early days, or Viking presently) has NEVER granted anyone approval to move the power levers aft of the idle stop in flight.

Use of Beta in flight is prohibited in 2.4.1.1(11) of the -400 POH, engine operating limitations table footnotes. I'm sure its prohibited in the -300 and earlier POH as well.

Absolutely correct. I know its correct, because I'm the guy who wrote the Series 300 & Series 400 flight manuals (at rev 53 and initial issue, respectively), and the FlightSafety DHC-6 training manuals (at rev 2 & 3).

...as I recall on the Twin Otter, 'beta range' is when the propeller rpm is controlled by the power levers, not the propeller levers...

That is correct. A simpler definition would be "whenever the actual propeller RPM is less than the RPM selected with the propeller levers, the propeller is in beta range". But, in the context of this discussion, I think that the original poster was referring to the practice of moving the power levers aft of the idle stop.

...one way they [FlightSafety] teach is if you have a steady beta light you are to power back, feather the prop pull the breaker....then bring it back.

No, that's not correct. FlightSafety teaches students to follow the procedure set out in the AFM, and what you stated is NOT the procedure set out in the AFM. Never was.

...So much easier to just twist the grip if you need a few extra seconds to find the breaker. ...Flight-safety won't instruct for these procedures so we took our own guy and just rented the sim for this training

I find that comment (about your group teaching your own procedures, those being to twist the power lever grip when presented with a steady beta light) a little difficult to believe, because when I was employed by FlightSafety and I did the initial certification of the DHC-6 simulator in 1992, we very deliberately programmed the simulator to freeze if someone twisted the power lever grips when the 'steady beta light' malfunction was active. We did this for the explicit purpose of preventing users (our own staff, or dry rental customers) from teaching unauthorized and unapproved procedures.

There is no need to select beta in flight for that approach with that 2100 foot runway in a Twin Otter. To need to do so it just poor approach planning and speed control.

Absolutely correct. The landing distance for a Twin Otter at maximum landing weight is 1,500 feet (ISA, no wind), and that's without reverse thrust. This assumes you cross the threshold at 50 feet AGL, at 1.3Vs. If you use reverse, the distance will be even shorter.

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

Pilots who have a flight test or engineering background will interpret the word 'beta' to mean "propeller operating at a speed less than what is selected with the PROPELLER lever". Twin Otter line pilots generally use the word 'beta' to refer to twisting the power levers and pulling them aft of the idle stop. It would be pedantic to use the engineering definition in a Twin Otter specific discussion, especially when the original poster made it clear he was referring to moving the power levers aft of the idle stop.

Pilot DAR explained things perfectly in his post above. But, obviously, if you are a DAR, you have a pretty strong flight test and engineering background!

Michael
(post edited to correct the typo that fellow forum member Cliff Jumper identified)

[Noo]

I've never seen so many egos murdered by words with one post.

[Cliff Jumper]

Thanks PanEuropean, it's nice to get info from the source.

Your points about 'beta' have me questioning my own understanding. You said the following two things...

"whenever the actual propeller RPM is less than the RPM selected with the propeller levers, the propeller is in beta range"

and

"propeller operating at a speed less than what is selected with the power lever"

My basic understanding of the term 'beta' in general, is any time the propeller is not providing forward thrust. I've understood 'beta range' to be any throttle position below flight idle. However, your two explanations have thrown me.

Your first definition would seem to include flight idle on the ground, where the propeller levers are full, throttles at idle, but prop rpm is less than maximum. I thought in this case there was still marginal forward thrust being produced by the props on the low pitch stops, and therefore not 'beta'.

Your second definition seems to alter this, unless its a typo. Any time you had a high throttle position, but selected a mid-rage propeller setting, this situation would exist, while the governor is governing. Heck even in reverse, how is the prop "operating at a speed less than what is selected with the power lever".??

Can you or DAR clear up this confusion for me?

[pelmet]

Thanks PanEuropean. I knew there was someone on here that was involved with this stuff but couldn't remember the name. I think it changed at some point.

I figured the guy didn't know what he was talking about and I knew you would have the answer.

Glad I followed the recommendations and never did such a thing myself.

Nice to cut through the BS.

[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 2171 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.