Beta Descent in the Twin Otter

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Hello

 

I was watching a You Tube video describing a 'Beta Descent' on a Pilatus in Papa New Guinea (I think).  

 

It occurred to me that, purely by trial and error, I have been adopting something similar in my Twin Otter to shed altitude when flying to small airstrips in the Himalayas where there is not much room for approach maneuvering.  I would put the pitch to minimum (100%) and throttle back and I found that I could descend quickly without gaining too much speed.  Then as I approached the airstrip, I could increase the power and land successfully.  

 

So, can you real world pilots tell me, is what I am doing a 'recognized' method of descending in mountainous terrain and is the real-world Twin Otter capable of doing this?  Are all prop aircraft able to perform beta descent (or similar)?   

 

I look forward to some 'real word' perspectives.

 

All the best

 

Mike

 

 

[CaptianJab 13]

 

On 11/14/2022 at 4:18 AM, expire said:

Hello

 

I was watching a You Tube video describing a 'Beta Descent' on a Pilatus in Papa New Guinea (I think).  

 

It occurred to me that, purely by trial and error, I have been adopting something similar in my Twin Otter to shed altitude when flying to small airstrips in the Himalayas where there is not much room for approach maneuvering.  I would put the pitch to minimum (100%) and throttle back and I found that I could descend quickly without gaining too much speed.  Then as I approached the airstrip, I could increase the power and land successfully.  

 

So, can you real world pilots tell me, is what I am doing a 'recognized' method of descending in mountainous terrain and is the real-world Twin Otter capable of doing this?  Are all prop aircraft able to perform beta descent (or similar)?   

 

I look forward to some 'real word' perspectives.

 

All the best

 

Mike

 

 

 

With the Twin Otter, you need to be careful using the word BETA. Because you are in Beta Range anytime the position of the prop levers and Np gauges disagree. So anything below 75 % Np.  This is also where power levers are controlling the propeller blade angle instead of the primary governor. 

 

Depending on who you ask descending with Props at 96% Np and the power levers at idle would be a perfectly acceptable way to descend because it is an authorized way to descend. compared to some planes that are authorized to use reverse in a descent.

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Thank you!  

[amahran 603]

14 hours ago, CaptianJab said:

 

 

With the Twin Otter, you need to be careful using the word BETA. Because you are in Beta Range anytime the position of the prop levers and Np gauges disagree. So anything below 75 % Np.  This is also where power levers are controlling the propeller blade angle instead of the primary governor. 

 

Depending on who you ask descending with Props at 96% Np and the power levers at idle would be a perfectly acceptable way to descend because it is an authorized way to descend. compared to some planes that are authorized to use reverse in a descent.

So if I understand correctly, the power lever should never be retarded in flight to the point that the governor is no longer limiting the propeller RPM (and that the situation where the prop RPM is controlled by the power lever position is a dangerous flight regime)?

 

I never quite understood why that was the case with turboprops. The only explanation I got in beta range dangers is that the “drag is high” (well of course it’s high, that’s why it’s tempting if to use it). What’s the actual danger associated with the use of Beta range?

[Aerosoft Team [Inactive Account] 51558]

Almost all aircraft that can do this in flight were allowed to do this when they were launched. However it really puts a massive amount of stress on the prop and the gearbox. It also sounds like the aircraft is about to come apart to pax.

So that is is why it is not SOP for most aircraft. 

[CaptianJab 13]

On 11/24/2022 at 2:15 PM, amahran said:

So if I understand correctly, the power lever should never be retarded in flight to the point that the governor is no longer limiting the propeller RPM (and that the situation where the prop RPM is controlled by the power lever position is a dangerous flight regime)?

 

I never quite understood why that was the case with turboprops. The only explanation I got in beta range dangers is that the “drag is high” (well of course it’s high, that’s why it’s tempting if to use it). What’s the actual danger associated with the use of Beta range?

No, the power lever should never be placed in a position where the propellers can go into reverse during flight. There is a mechanical lock to stop that from happening. For example, on the Twin Otter the Power Levers physically cannot go into reverse unless the Propeller Levers are in the full Forward position and the grips on the power levers are twisted. Then the mechanical lock on the power levers will release allowing you to go into reverse. 
 

Remember, Beta Range is when the primary governor is no longer controlling the pitch of the props. At this point the props are controlled by the power levers. Therefore anytime on an approach or on the ground, once the prop levers are set to max but the Np gauge doesn’t show 96%, you are in beta range which is normal. 

 

With all of that being said, once you aren’t in reverse during an approach you’re fine.