I just planned and flew a flight from Zurich to Innsbruck in the PMDG 737-700 using Simbrief as my flight planning tool. The flight level given from Simbrief for cruising was FL170. When ready to taxi from the gate, the FMC in the 737 was suggesting an Optimum flight level  of FL386. Now that is one hell of a difference. What happens in the real world - and which, if any is the correct cruise level to use - both for economy and VNAV functionality, particularly with what was a very short flight. Just do not thing a flight level of anything above FL250 would have worked.

3 minutes ago, SquadronLeader said:

I just planned and flew a flight from Zurich to Innsbruck in the PMDG 737-700 using Simbrief as my flight planning tool. The flight level given from Simbrief for cruising was FL170. When ready to taxi from the gate, the FMC in the 737 was suggesting an Optimum flight level  of FL386. Now that is one hell of a difference. What happens in the real world - and which, if any is the correct cruise level to use - both for economy and VNAV functionality, particularly with what was a very short flight. Just do not thing a flight level of anything above FL250 would have worked.

The "optimum" flight level is the flight level with the lowest fuel consumption (restricted by aircraft weight). It will take into consideration things like your weight, the cost index, ISA temps etc.. It will not consider if you can realistically reach that flight level before you have to descent again, because those decisions are influenced by multiple factors (SID going to the opposite direction [on some SIDs you might already be at FL 170 when you start turning towards Innbruck at all], ATC clearances, speed restrictions by ATC for separation etc.) that the FMC can not know. It will only tell you where you will burn the lowest amount of fuel.

A proposito: Simbrief's flight levels for short flights are also mostly wrong for the same reasons. I have been giving flight levels on Simbrief I would never be able to reach before the TOD, and I've been giving flight levels where I would have been cruising along 20 minutes before the TOD.

So on short flights just enter all the data into the FMC and take a look at the altitude predictions on your route. You will see much more precisely what altitude you can make, because the FMC will (in contrast to Simbrief or the optimum calculation) consider the actual routing and winds. Then adjust the cruise altitude on the perf init page accordingly.

Well it depends on what you are doing and how long you plan on flying. If optimum is FL400 and I'm only going a short distance, I won't climb to FL400 just because it's optimum. I might not even be able to get up there and back down before I need to descend. Also, ATC constraints may require that I stay at a lower altitude. Now, if I am doing an ocean crossing or going a long distance, I will hit optimum to save fuel. I might have to fly at optimum altitudes to make the trip with the fuel load. I may have so much gas that I stay low so I don't have to suck the mask when the other person is out of the seat.

I tend to check flight aware website for similar flights and use the published altitudes that were used by those flights. You can always request a higher (or lower) altitude once you are up there.

1 hour ago, SquadronLeader said:

I just planned and flew a flight from Zurich to Innsbruck in the PMDG 737-700 using Simbrief as my flight planning tool. The flight level given from Simbrief for cruising was FL170. When ready to taxi from the gate, the FMC in the 737 was suggesting an Optimum flight level  of FL386. Now that is one hell of a difference. What happens in the real world - and which, if any is the correct cruise level to use - both for economy and VNAV functionality, particularly with what was a very short flight. Just do not thing a flight level of anything above FL250 would have worked.

Please double check all the waypoints on the simbrief fligtplan to see if at some point you are supposed to climb to the optimal flight level. 

The last time that happened to me i noticed that after a particular waypoint with the low cruise FL the other waypoints had increasing flight levels until i got to the optimal cruise later in the flight.

5 minutes ago, FrankR409 said:

I tend to check flight aware website for similar flights and use the published altitudes that were used by those flights. You can always request a higher (or lower) altitude once you are up there.

Exactly.  The airlines pay people to do this so I just punch in my flight into Flight Finder on Flightaware and use whatever FL the real flights are using that day. 

29 minutes ago, MDFlier said:

Exactly.  The airlines pay people to do this so I just punch in my flight into Flight Finder on Flightaware and use whatever FL the real flights are using that day. 

This would only make sense if you fly at the same time. Already 6 hours later things could be quite different in regards to winds or cloud bases.

1 hour ago, FrankR409 said:

I tend to check flight aware website for similar flights and use the published altitudes that were used by those flights. You can always request a higher (or lower) altitude once you are up there.

Sure, flightradar24 too is a good source for real world flight altitudes, if there is a history of a real world flight you can get the data on your route.

It's well worth setting up a simple ADS-B data tracking station and submitting there days to FlightAware, FlightRadar24, etc so you qualify for free Business class accounts on the services (you can submit data from one computer to multiple services so your don't need more than one tracker)

Do not that, as discussed, the optimum flight level does not factor in winds, so the 'true' optimum flight level can be another because of more favorable winds that would yield better fuel results than the FMC calculated optimum level in that instance. This is what OFPs factor in when the cruise level is calculated.

2 hours ago, Fiorentoni said:

This would only make sense if you fly at the same time. Already 6 hours later things could be quite different in regards to winds or cloud bases.

That's exactly what I do. If there isn't a departure between the 2 cities I chose within an hour or so, I reverse the route and check it. It's pretty much a rule for me that I track a live flight. I like seeing how close I can get to their departure and arrival times and their people are doing my planning for me.