This was popular in the early years, to exchange the mostly air based Bendix FCU for the mostly hydraulic Woodwards. The Bendix had issues with moisture freezing in the system causing problems. There was some sort of incentive program to make the switch in the late 1970s.

My airplane, originally -6 which is basically the same as a -5 when it comes to FCU, came with the Bendix fuel controller. It was converted, via SB TPE331-72-0120 and STC SA2202SW, to Woodwards in 1978, 3 years after it was built.

Might check out STC SA1024SO, seems to be swapping out Bendix for Woodward FCUs on -5 equipped 690/A/B.

The Woodwards were the subject of an AD, 2006-15-08, related to fuel spline failures. This resulted in runaway engines, above 100% torque. Honeywell's fix was to change the fuel spline from steel to plastic and to modify the FCUs to fail low instead of fail high. The cost of this mod was very high, $40K per side, and owners felt the fail high was safer. An AMOC was put in place that required 1000 hour inspection intervals of the fuel spline. This AMOC was later incorporated permanently into the AD by revision. This saved the TPE331 community about $100M on total. I was part of the meetings with the FAA on getting the AMOC approved.

Ironically, the Bendix didn't get an AD on them despite the reputation they have for trouble. A few Bendix FCU engines still exist out there but they are the minority.

Also, for the -10 upgrade, you must have Woodwards.

Mike C.

I think the cost is 40k for both. I had mine done at overhaul.

Having an engine go to full throttle if the FCU failed, seemed dangerous particularly on a twin engine plane

The fix is to have the engine go to 180pph which would be much easier to control.

Single engine I would take the full throttle failure but not in a twin.

Full power on a turbine would likely melt the turbine and either sling out the prop blades, internal engine blades, or some other nasty thing. I've heard of sub 1000 HP turbines experiencing failure modes that produce several thousand HP for a few seconds before self destructing.

Buy one, have an FCU problem with it where you would like to shut that engine off, then you will say you should have bought a Commander.

Mike C.

Buy one, have an FCU problem with it where you would like to shut that engine off, then you will say you should have bought a Commander.

Mike C.

I disagree with you quite strongly on this. Dozens of simulated FCU failures in the sim by myself and others reinforce my opinion that I prefer an FCU to fail high.

One problem is that fail low, to 180 pph or there about, does nothing for the pilot. That is producing less than flight idle power, so it puts the engine in a high drag place with no NTS. Yet the engine is still operating. This is confusing and useless.

In contrast, the fail high FCU takes you to 130% power, roughly. If this occurs on takeoff, the thrust asymmetry is only 30% which is trivial to handle. This gives you time to climb out away from the ground, accelerate, and then feather the truly faulty engine.

What happens on approach? It may seem dangerous there to have one engine go to high power, but that turns out to be fairly easily handled as well. The thrust asymmetry is roughly 90% (40% good side, 130% bad side). The multiple pilots and tries in the sim show this is a non issue and there are at least 2 SDR reports of this happening on approach with no accidents.

Here is a fatal accident with a fail low FCU on approach:

http://www.tsb.gc.ca/eng/rapports-repor ... 3c0150.pdf

Left engine goes to near zero torque (which is pretty high drag) even though commanded to near 100% by the pilots. The engine does NOT NTS. Table 4 gives this data in the report. The crew is confused, and don't feather. This leaves the left engine in a high drag situation that they do not recover from. 5 people died, 2 survive.

Had the FCU failed high, this accident does not happen.

On a twin, I want fail high FCUs. Absolutely.

The only time a fail low FCU is worth anything is when you are still on the start locks. It may then prevent an engine from overspeeding to destruction. Once off the locks, the prop governor will prevent the engine from overspeeding.

The last issue is that the new style FCUs with fail low behavior has Vespel (plastic) fuel splines. They are failing at a high rate in the field. The old style metal (stainless steel) are not. Honeywell says the old style have had about 50 failures, but the VAST majority of these are outside the US with planes using questionable fuel (mostly ag planes using ag diesel). The risk of a fuel spline failure when using proper fuel is virtually nil.

Mike C.

That does not happen on the TPE331. The capacity of the fuel pump and fuel nozzles is such that you only achieve about 130% power on FCU failure for fail high.

For the -10 engines with good temperature margins, this may not even put you over temp limit.

I know of a case of a freshly overhauled TPE331-10T that suffered a fuel spline failure on its first flight (latent defect). Some time after climb out on the test flight, engine went to 130% torque, temp went slightly over limit, like 20-30C over. Pilot calmly feathered it and came back and landed OEI. The engine was borescoped, given a clean bill of health, and has now flown about 1500 hours since with no discernible difference to its twin on the other wing.

Note that when this happens, the engine does NOT overspeed. This is high torque, not high RPM. The prop governor will load the engine so that RPM is controlled.

Mike C.

I'd like to see that done with an engine well over 100% power as an FCU runaway can cause. You be past gear speed, past flap speed, and possibly past Vmo as well, when getting over the numbers.

Mike C.

I disagree with you quite strongly on this. Dozens of simulated FCU failures in the sim by myself and others reinforce my opinion that I prefer an FCU to fail high.

One problem is that fail low, to 180 pph or there about, does nothing for the pilot. That is producing less than flight idle power, so it puts the engine in a high drag place with no NTS. Yet the engine is still operating. This is confusing and useless.

In contrast, the fail high FCU takes you to 130% power, roughly. If this occurs on takeoff, the thrust asymmetry is only 30% which is trivial to handle. This gives you time to climb out away from the ground, accelerate, and then feather the truly faulty engine.

What happens on approach? It may seem dangerous there to have one engine go to high power, but that turns out to be fairly easily handled as well. The thrust asymmetry is roughly 90% (40% good side, 130% bad side). The multiple pilots and tries in the sim show this is a non issue and there are at least 2 SDR reports of this happening on approach with no accidents.

Here is a fatal accident with a fail low FCU on approach:

http://www.tsb.gc.ca/eng/rapports-repor ... 3c0150.pdf

Left engine goes to near zero torque (which is pretty high drag) even though commanded to near 100% by the pilots. The engine does NOT NTS. Table 4 gives this data in the report. The crew is confused, and don't feather. This leaves the left engine in a high drag situation that they do not recover from. 5 people died, 2 survive.

Had the FCU failed high, this accident does not happen.

On a twin, I want fail high FCUs. Absolutely.

The only time a fail low FCU is worth anything is when you are still on the start locks. It may then prevent an engine from overspeeding to destruction. Once off the locks, the prop governor will prevent the engine from overspeeding.

The last issue is that the new style FCUs with fail low behavior has Vespel (plastic) fuel splines. They are failing at a high rate in the field. The old style metal (stainless steel) are not. Honeywell says the old style have had about 50 failures, but the VAST majority of these are outside the US with planes using questionable fuel (mostly ag planes using ag diesel). The risk of a fuel spline failure when using proper fuel is virtually nil.

Mike C.

I'm sorry, you are correct.

What I should have said is that the Red Lake accident has the same effect as an FCU fail low failure. Engine operating, but in high drag, no thrust. RPM was near 100%, torque was zero or even minus a few percent, fuel flow and temperature were indicating operation.

Mike C.

Your plane does not have the optional pre-select system. Most 690A's do not. It's retrofitable but likely not worth the expense. I suspect a 2100 DFCS AP install would run you just north of $100k when it's said and done fwiw.

If I set 180pph on approach to simulate a FCU failure the affect is almost nil!

If I go to 130% power on one engine on a approach the affect would be dramatic.

Easy decision for me.

Typical flight idle fuel flow is more than 180 pph. So how are you getting to that fuel flow level in the sim?

There is absolutely no advantage to keeping the engine operating at such a low fuel flow. It would have been better if the FCU failure simply flames out the engine instead of this coma state. Then there is nothing the pilots have to know about FCU failure, it looks like any ordinary engine failure.


Not really. It is FAR less dramatic than an engine failure on takeoff where the thrust ratio is 100%/-20% and that occurs at less airspeed. On approach, having a thrust ratio of 35%/130% is not that difficult and it occurs at higher airspeed. It is natural to feel the power change, put in proper controls, decide to go missed, and bring the power up on the good engine and go around.

I don't know if the Commander sim has FCU fail high option (MU2 sim has both FCU fail high and low), but if it does, try it out, it is not that difficult, especially for a Commander with such benign engine out behavior.


Every MU2 pilot who thought as you did changes their mind after I take them through the sim and show the two failure modes in the various flight regimes. The benefit on takeoff is substantial increase in safety, handling it on approach is not a big deal.

Here is an SDR report on landing:

CA010817001 (2001)

AIRCRAFT WAS ON APPROACH AND THE RIGHT HAND ENGINE WENT TO FULL POWER
AND WENT INTO BY-PASS. THE PILOT TRIED TO RETARD POWER BUT THERE WAS NO
EFFECT. PILOT SHUT THE RIGHT HAND ENGINE DOWN AND LANDED WITHOUT
INCIDENT. FCU AND PUMP WERE REPLACED AS AN ASSEMBLY.

This was in Canada, a Metro, same model that crashed in Red Lake.

We got 40+ years running on the fail high FCUs with NO injuries or fatal accidents. That is better testing and evidence than anything else. The new design fail low FCUs are failing in the field at a rate higher than the old style, BTW. This is why the FAA rescinded the requirement in the AD to get them modified, now replaced with a repetitive spline inspection so the old design can stay in service.

Mike C.

The procedure for a modified FCU failure is the same as an engine failure. Simple.

With a un modified FCU it's different. You get more than full throttle on one engine while the other is at a very low setting.

You are also relying on the limiters to bypass enough fuel to keep the engine from self destructing.

The limiters were designed to bypass fuel with a good FCU not a failed one.

Your examples in the sim are just that examples. You were expecting the failure!

Imagine it happening with no warning.

We have choices, I undstand the affects of the modification and I balanced the cost versus the increase safety.

Obviously I think the modified FCU is safer.

I'm sorry, you are correct.

What I should have said is that the Red Lake accident has the same effect as an FCU fail low failure. Engine operating, but in high drag, no thrust. RPM was near 100%, torque was zero or even minus a few percent, fuel flow and temperature were indicating operation.

Mike C.