Schrijfsels Document : f50_emerDC.txt

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Strange behavoure with Essential-DC bus failure on Fokker 50
By Pascal Schiks.

In the past during training sessions several instructors have discovered a odd behavoure off the EngineElectronicControl during ESSential-DC bus failure.
While simulating this failure, pilots reported a MAN mode light off the EEC on
the off side EEC.

Here I try to describe what and why is going on.

Problem description :
Having an Essential-DC failure is quiet an cumbersome event, as with the loss of some avionics also the power to the EEC and engine instrument on the failed side is lossed.
The engine will continue to run but it will do so without EEC and therefore run in MANual mode.
You might expect an MAN light on the EEC panel, but for iluminating the bulb one needs electrical power which is unavailable due to the loss of the onside ESSential-DC bus.
This is all fairly normal behavoure.

However, also more unexpecingly the offside EEC will switch to manual mode.
This behavoure was introduced by an ammended modification by Fokker.
During simulator training was discovered that during single engine aproach
with onside failed ESS-DC bus, the flightcrew was not able to Go Around this due to the lack of response off the EEC.
Therefore decided was that in such an event Automatic MANual mode selection is required.
This is archieved by a relay called the Fail Fixed Relay.

Next problem is that is a twin engine situation, still with a ESS-DC bus failure,
there is no way of getting engine data due to the loss of the instruments.
Fokker came up with a simple but adequate solution to overcome this problem.
a second relay (Auxilary Fail Fixed Relay) was introduced which will force the offside engine into manual mode.
As a result of this, the offside MAN light will iluminate.

The idea is that the offside engine like the onside engine running in MANual mode has its full range of instruments still available.
By keeping both powerlevers in equal position and monitoring the offsite engine instruments the crew now has an accurate guess of the engine performance on both sides.
Unfortunately not al training manuals where updated on this feature, with ignorance of most flightcrews and instructors as a result.
In corresponding with Fokker about this mather, the company replied,
that airline companies should have updated their manuals on this issue.
Since the Fokker company by then already was close to it's end these probarly where things that did not had priority.