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Ram air TIO540

I maintain a TB21 and I Inspect Annually another. The system is in fact pure Lycoming and is identical to the turbo installation in a re-start Cessna T182.

It is an automatic controlled wastegate system. I did not find the set-up to be particularly delicate, but I did witness 3 turbo main bearing failures (followed by the turbine disintegrating ) in less than 400 Hours TIS . One of the turbos failed at 50H TIS. The actual CAUSE of the bearing failures has never been determined and remains an egnima – very frustrating to say the least.

FAA A&P/IA
LFPN

Michael wrote:

Automatic controlled wastegate : of which there are several types. Suffice it to say that an automatic controller maintains a manifold pressure the pilot asks for through throttle position. It’s largely a set-and-forget system and reduces pilot work load tremendously.

That is the case in Timothy’s case, and as I posted above, the Navajo seems to be particular because it is one of the few types that has a differential pressure controller and a density controller. That is why I think it does not “suffice to say” in this case that the pressure controller always maintains the pressure commanded by throttle position, because that hasn’t changed in Timothy’s case, and MP still did vary.

Allow me to quote my quote from the above post again:

In the Piper, the density controller modulates the wastegate movement at wide open throttle while a differential controller keeps deck pressures from exceeding manifold pressures by more than a specified amount at part throttle settings.

Flying with WOT, or according to the book Fly the Engine “at high power settings – 90 percent or or better – the differential pressure controller’s action is superseded by that of the density controller, a sealed aneroid device containing a charge of dry nitrogen. Note that whereas the plain-Jane absolute pressure controller contains only a vacuum-purged, spring-loaded aneroid (capable of sensing pressure, but not temperature), the density controller’s aneroid is gas-charged and thus sensitive to both temperature and pressure (hence density). It’s really quite clever, when you think about it: a controller that compensates for density altitude.”

Hence we need to know at what power settings Timothy was flying when he saw this MP change. Then we can know which pressure controller might be involved in the change. But even in the WOT case, it would be a wild guess to say why a change in airspeed would cause such large variation in the air density sensed by the pressure controller.

It was at 65% LOP, so certainly nowhere near full throttle.

It all seems to need a lot of very careful balancing, especially LOP, which I would attribute to a poorly maintained controller if it were not for both engines behaving identically despite a totally different history.

I flew a Chieftain a couple of days ago, but only for a few minutes at low altitude, but the feel was very similar, despite rather different engines (also TIO540s, but 350hp as opposed to 310.)

EGKB Biggin Hill

This kind of reminds me of a PA28RT-201T I used to fly in California.

It had a Continental TSIO360. The MAP limit was 40 InHg. If I set 40" at the beginning of the takeoff run, during the acceleration down the runway the MAP would increase sufficiently for the overboost warning to come on (at 42" IIRC). The MAP was also extremely sensitive to throttle adjustments. I have never since flown a turbocharged airplane which was that sensitive on the throttle, nor any other airplane of same model to have an idea of whether this characteristic was typical of that engine or whether there was something wrong with the engine.

LFPT, LFPN

Well, I have a TB21 and MP is not just controlled by the throttle. whenever I change the prop pitch my MP changes also so there is a link between the prop pitch and MP.

LSZH

placido wrote:

I have a TB21 and MP is not just controlled by the throttle. whenever I change the prop pitch my MP changes also so there is a link between the prop pitch and MP

On most engines this ‘link’ results from the slight change in pressure drop along the intake tract (and throttle plate) when the intake air flow changes with RPM. It’s a second order effect, not a primary or intended result of engine RPM control. Is there something about the turbocharged TB21 engine that is different than that?

placido wrote:

Well, I have a TB21 and MP is not just controlled by the throttle. whenever I change the prop pitch my MP changes also so there is a link between the prop pitch and MP.

There is no physical “link” between the prop pitch and turbo system.

The Lycoming TIO-540 system installed in the TB-21 has an automatic waste-gate controller that includes a density controller.

The most important and least understood aspect of this system is that the controller strives to maintain a specific Upper Deck Pressure ( UDP ) regardless of what the throttle position is. Since the throttle plate creates a restriction, slight when wide open, much greater as the throttle is closed, the actual Manifold Pressure is always less than the UDP. Since there is no gauge connected to the UD, the pilot has no way of knowing what the actual UDP is at any given time. That said , the UDP is usually set-up to 2" to 4" Higher than the Maximum MP that is specified for the engine.

Under operation, the following can be observed:

- When the engine is started and idling (throttle plate practically closed) the UDP will be very low so the Controller will close the wastegate in order to try to boost the UDP to it’s pre-determined pressure let’s say 36" in this example.

- As the throttle is opened, the engine makes more HP and expels more hot exhaust gasses into the turbo thus boosting the UDP.

- At some point, the engine is making sufficient power (hot exhaust gasses) that the UDP is attained, at this point if the throttle is advanced even more, the controller will start opening the waste-gate to dump excess exhaust gasses since the UDP has been achieved.

- From now on, the controller will open or close the waste-gate solely as a function of maintaining the predetermined UDP of 36" .

- Generally speaking, at or near Sea Level, there is far more HP available at 3/4 throttle and more to maintain the predetermined UDP, so the waste-gate is usually open venting excess exhaust gasses.

- As the aircraft is climbed, the turbo charger needs to work harder to compress the lower density air to produce the UDP.

- At some point the turbo charger is no longer able to produce enough energy to maintain UDP. This point is called the Critical Altitude.

FAA A&P/IA
LFPN

Silvaire wrote:

On most engines this ‘link’ results from the slight change in pressure drop along the intake tract (and throttle plate) when the intake air flow changes with RPM. It’s a second order effect, not a primary or intended result of engine RPM control.

What the TB-21 owner/pilot is seeing is the second order effect of the prop load changing creating a change in HP which in turn creates a change in UDP . If the waste-gate is closed when this occurs, then the change in UDP will have a direct effect on the MP.

My guess is that this phenomena would obe observed WOT and at higher altitudes where the waste-gate is usually fully closed.

Is there something about the turbocharged TB21 engine that is different than that?

No.

Last Edited by Michael at 30 Apr 16:09
FAA A&P/IA
LFPN

Can the UDP be measured? It would seem that having this pilot-readable (perhaps as a parameter on an engine monitor) might aid diagnosing turbo issues, before they become critical.

Administrator
Shoreham EGKA, United Kingdom

Peter wrote:

Can the UDP be measured?

Of course it can, but I doubt that this info would be of any particular use to the pilot.

Whats most important is to understand how the system works.

FAA A&P/IA
LFPN
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