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    Simultaneous engine start


    Tango777
    • Status: Pending

      717 Version: 1.0.9.22

      System Category: Air

      Minidump: https://forums.tfdidesign.com/uploads/

    The engines can still start at the same time while running off of the APU.

    Suggestion: Program the packs so that when we turn the packs off for engine start the APU can only supply enough bleed air for 1 engine to start. Therefore if we pull on both engine start knobs at the same time neither engine would be able to start. This would lead to only starting 1 engine at a time which is what should happen. Off the top of my head the simultaneous engine starts, (2 engines at 1 time) while on the APU, only exist on the B777, B747, and B787 to the best of my knowledge. I do not know about the A350 or A380.

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    On 3/7/2018 at 11:31 PM, turbofandude said:

     @Mistrfiksit how would the aircraft behave under these conditions?

    I really don't have enough info for this because it's not a normal practice to start both engines simultaneously. I can tell you what could happen by logic and design though.

    One thing I noticed in the sim, is that there is no drop in manifold pressure when you select one engine to start. The APU will throw all the air it can in a MES (main engine start) by opening the bleed valve all the way. But there will be a significant drop in pressure during start regardless. Minimum start pressure is 35psi.

    Given that, in RW, with both starters engaged the PM could drop below 25psi, which would trigger a START AIR LO alert. The engines would never make max N2 motor, and if the fuel switch was thrown (and the EEC allowed fuel to enter, min. N2 is 19 or 20%) it could lead to a hot or hung start. And the APU EGT could increase to the point that if it overtemped, it would close the bleed air valve and stopping any engine start altogether.

    That's just a few things that could happen but there's possibly more.

    Edited by Mistrfiksit

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    1 hour ago, Mistrfiksit said:

    I really don't have enough info for this because it's not a normal practice to start both engines simultaneously. I can tell you what could happen by logic and design though.

    One thing I noticed in the sim, is that there is no drop in manifold pressure when you select one engine to start. The APU will throw all the air it can in a MES (main engine start) by opening the bleed valve all the way. But there will be a significant drop in pressure during start regardless. Minimum start pressure is 35psi.

    Given that, in RW, with both starters engaged the PM could drop below 25psi, which would trigger a START AIR LO alert. The engines would never make max N2 motor, and if the fuel switch was thrown (and the EEC allowed fuel to enter, min. N2 is 19 or 20%) it could lead to a hot or hung start. And the APU EGT could increase to the point that if it overtemped, it would close the bleed air valve and stopping any engine start altogether.

    That's just a few things that could happen but there's possibly more.

    Alright - would a reasonable choice be to reduce offside manifold pressure while starting? (i.e. Left starter is engaged, so reduce right-side pressure, as there won't be as much air). This would accomplish not being able to start both simultaneously.

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    3 hours ago, turbofandude said:

    Alright - would a reasonable choice be to reduce offside manifold pressure while starting? (i.e. Left starter is engaged, so reduce right-side pressure, as there won't be as much air). This would accomplish not being able to start both simultaneously.

    It wouldn't be realistic since the pressure in the ducting would all drop unless the ISOL valve were closed and the right side were engaged. However, it looks like there is already logic in place when it comes to having both packs running and trying to start a single engine. The engine doesn't increase N2 and you get the START AIR LO message. Could this be coded in the same manner as trying to start both engines at the same time? If one starter is engaged and the other is then selected, it could behave this way too. Pressure drops low,  none or little N2 movement and the message appears. Basically what Tango777 suggested. Also, for the normal way, have the PM drop about 3-8psi (good round number) when the starter engages and the engine rotates.                        

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    @Mistrfiksit I totally over complicated this. I agree - I'll have the pressure drop in both manifolds when a starter engages. Pulling both will cause the pressure to drop too much to start either one.

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