Hi

 

I can recall being able to rely on the VNAV cruise step climb predictions of pmdg 747-4.

 

But now flying 747-8, even though i here its same flight model, VNAV cruise step climb predictions seem very errornous.

 

I am 35,000, VNAV cruise says its ok to climb to "41K AT NOW" when i attempt to do that using VNAV climb a/c nearly stalls when it reached 38K.

 

Any ideas.

Hello

 

What is OAT?

 

Ionut "John" Micu

Are you using the PMDG Load Manager and have you correctly selected the correct 747-800 from the config menu?

 

 

 

Hello

 

What is OAT?

 

Ionut "John" Micu

 

Outside Air Temperature

 

But why post that in this thread? It has nothing to do with it

Are you using the PMDG Load Manager and have you correctly selected the correct 747-800 from the config menu?

 

 

 

fstulm, on 17 December 2012 - 08:24 AM, said:

Hello

 

What is OAT?

 

Ionut "John" Micu

 

Outside Air Temperature

 

But why post that in this thread? It has nothing to do with it

If the performance characteristics of both the B747 and B747-8 add-ons are the same, selecting the correct variant should not make a difference. I believe Ionut knows what OAT is, and merely missed a "the". I'm guessing that OAT may affect VNAV behavior.

 

In any case, you could try running the Flight1 Registry Repair Tool in order to eliminate any installation/software issues.

I believe Ionut knows what OAT is, and merely missed a "the". I'm guessing that OAT may affect VNAV behavior.

 

 

Well excuse me for not picking that out. Yes OAT affects VNAV behaviour, however the OAT and many other conditions and characteristics are already taken into account when calculating step climbs, so this shouldn't be an issue unless there is some really bizzare OAT.

 

Another thing to consider is whether the PMDG Load Manager and the aircraft itself are using the same measurement system (LBS OR KGS) If you are boarding fuel and payload in LBS are your aircraft or FSX is using KG'S this could potentially cause problems

If you are boarding fuel and payload in LBS are your aircraft or FSX is using KG'S this could potentially cause problems

But I'm wondering if this can actually affect the accuracy (i.e., confuse) the flight management computer. It can certainly affect aircraft performance, but will it cause calculation errors in and invalidate the logic of the FMC? It's difficult to explain what I'm suggesting, so I'll try again in the morning if you find my statement confusing.

But I'm wondering if this can actually affect the accuracy (i.e., confuse) the flight management computer. It can certainly affect aircraft performance, but will it cause calculation errors in and invalidate the logic of the FMC? It's difficult to explain what I'm suggesting, so I'll try again in the morning if you find my statement confusing.

 

Well perhaps you might like to suggest some solutions of your own instead of attacking mine.

Hello all,

 

Thank you Owen, I did miss the "the" in "What is the OAT?"

 

So, that question again, what OAT do you see at the altitude where you want to do a step climb?

 

OAT does not affect VNAV, VNAV functionality is just fine regardless of temperature; OAT affects the aerodynamic performance of the aircraft.

 

I have observed in various threads a similar situation and faulty weather was to blame.

 

Regards,

Ionut "John" Micu

Well perhaps you might like to suggest some solutions of your own instead of attacking mine.

I'm only curious as to whether the FMC directly interprets the data from the configuration tool in some sort of static unit.

 

I use hybrid units in FSX and imperial units with the PMDG B747, but experience no VNAV issues.

I'm only curious as to whether the FMC directly interprets the data from the configuration tool in some sort of static unit.

 

The FMC receives nothing from the config tool, directly. It reads the load from the aircraft.cfg for the predicted weights, but that's it.

 

The reason someone mentioned it is that the FMC's logic will either overperform or underperform if you give it weights that are inaccurate. As an example, if the FMC is expecting LBS and I give it the numbers I was handed in KGs (roughly half of what LBS would be), it will assume it's very light and assume that it can climb high, climb quick, and fly far. That isn't the case, however. The real case is that someone gave it the wrong units.

 

If I can cruise at 35000 with a load of 40000 (fake values here), but I tell it my load is only 20000 (because I'm looking at a load sheet in KGs, when it's expecting LBS), it will assume it can go higher than 35000.

 

 

 

Additionally, to whoever is involved in the OAT discussion (other than the OP and the John):

First, for what it's worth, I did pick up that he intended the question to mean "what is the OAT," but that's probably because I know John tends to know what he's talking about. Easy mistake.

Second, OAT has a lot to do with it. OAT does affect cruise altitudes (this is why you specify the ISA DEV on the PERF page). Also remember that certain weather programs used with the sim provide very erroneous temp values at higher altitudes. These errors are normally on the high temp side, but could be very cold as well, tricking the logic.

The FMC receives nothing from the config tool, directly. It reads the load from the aircraft.cfg for the predicted weights, but that's it.

 

The reason someone mentioned it is that the FMC's logic will either overperform or underperform if you give it weights that are inaccurate. As an example, if the FMC is expecting LBS and I give it the numbers I was handed in KGs (roughly half of what LBS would be), it will assume it's very light and assume that it can climb high, climb quick, and fly far. That isn't the case, however. The real case is that someone gave it the wrong units.

Thanks for the technical clarification. Wouldn't it be difficult to input values with incorrect units into the FMC, because the fuel and gross weights are automatically calculated, at least in the PMDG rendition?

Hello all,

 

Thank you Kyle, that's exactly what happened

 

As for the OAT possible issue, I was waiting to see what is the actual temp deviation that the user is experiencing and then delve into the technical explanation that you very well beat me to it.

 

Merry Christmas all,

 

Ionut "John" Micu

Thanks for the technical clarification. Wouldn't it be difficult to input values with incorrect units into the FMC, because the fuel and gross weights are automatically calculated, at least in the PMDG rendition?

 

You're welcome.

Yes and no. I don't use auto calcs for my weights. I hand enter ZFW, and occasionally fuel (either for expected fuel, or because the actual amount differs slightly from the FMC calc). Someone who just accepts the auto calc value would be mostly okay, however, if they're looking at a load sheet in KG and see the LBS, they may try to override it and hand enter the LBS value, which would mess it up.

 

In the real world, a pilot really wouldn't come across that, especially in today's day and age, where flight deck ergonomics and sense are so important. It's already caused one incident up in Canada (the Gimli Glider), because the fleet was on imperial weights, and then they converted to metric. Habit and perceived expectations are a hard thing to kick.

 

Thank you Kyle, that's exactly what happened

 

As for the OAT possible issue, I was waiting to see what is the actual temp deviation that the user is experiencing and then delve into the technical explanation that you very well beat me to it.

 

You're welcome.