Short story: PERF INIT includes ice weight into ZFW when you use the LOAD button, causing all related calculations to be wrong, despite some CDU displays correct ZFW.
Long story:
I flew from an icing airport to a non-icing one.
when I was pre-flight I noticed the ZFW in FUEL menu is highlighted, and it shows 300kg more than ZFW in PAYLOAD menu.
I continued preflight, but kept monitoring the disagreement.
As I taxi to runway, I noticed "Icing" in PAYLOAD menu went from "none" to 1.3% then to 1.4%, maximum was 1.7%, and the disagreement between FUEL ZFW and PAYLOAD ZFW increases.
After take off, the icing percentage immediately began to drop, but it seems to show anything less than one whole percent as "none". the disagreement between the ZFWs drops, and finally reaches zero (both PAYLOAD and FUEL shows same ZFW).
When I was about to land I noticed something wrong with landing Vref settings. I am very used to land the plane around MLW, so I know the Vref should drop from 30/150; 40/143 to 30/149; 40/142 right at 66.7 gross weight (which is IIRC the MLW for 738).
But it didn't. I waited on and it finally did at 66.4 gross weight.
The 300kg difference sounded familiar so I checked the ZFW setting in PERF INIT, and sure enough, it was 300kg more than the ZFW shown in both FUEL and PAYLOAD menus (as there's zero ice now, both shows correct ZFW). However, the Gross Weight display in Landing Vref menu was correct.
So that leads us back to the "nutshell version": PERF INIT incorrectly includes ice weight when inputting your ZFW, causing the FMC to think that your plane is heavier than it actually is.
The aircraft involved is 737-800 but I would guess that this issue affects all MSFS PMDG 737 family.
Long story:
I flew from an icing airport to a non-icing one.
when I was pre-flight I noticed the ZFW in FUEL menu is highlighted, and it shows 300kg more than ZFW in PAYLOAD menu.
I continued preflight, but kept monitoring the disagreement.
As I taxi to runway, I noticed "Icing" in PAYLOAD menu went from "none" to 1.3% then to 1.4%, maximum was 1.7%, and the disagreement between FUEL ZFW and PAYLOAD ZFW increases.
After take off, the icing percentage immediately began to drop, but it seems to show anything less than one whole percent as "none". the disagreement between the ZFWs drops, and finally reaches zero (both PAYLOAD and FUEL shows same ZFW).
When I was about to land I noticed something wrong with landing Vref settings. I am very used to land the plane around MLW, so I know the Vref should drop from 30/150; 40/143 to 30/149; 40/142 right at 66.7 gross weight (which is IIRC the MLW for 738).
But it didn't. I waited on and it finally did at 66.4 gross weight.
The 300kg difference sounded familiar so I checked the ZFW setting in PERF INIT, and sure enough, it was 300kg more than the ZFW shown in both FUEL and PAYLOAD menus (as there's zero ice now, both shows correct ZFW). However, the Gross Weight display in Landing Vref menu was correct.
So that leads us back to the "nutshell version": PERF INIT incorrectly includes ice weight when inputting your ZFW, causing the FMC to think that your plane is heavier than it actually is.
The aircraft involved is 737-800 but I would guess that this issue affects all MSFS PMDG 737 family.
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