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captain420

What are step climbs for and how to do it?

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I heard about this and wanted to know exactly what a step climb is used for? Is it automatically calculated into your FMC when uploading or entering in your routes? or is this something you have to manually specify?

 

Also in the PMDG 777, how would you manually perform a step climb? I'm assuming using altitude intervention mode? Ex: dial in the desired altitude on the MCP and hit the FLCH or VS button? Correct me if I'm wrong. Or is there a better way to handle step climbs?

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Hi Aaron,

 

A step climb is simply a way of keeping the aircraft at (or near) the optimum altitude at any given moment in the flight. The optimum (i.e. most fuel-efficient) altitude is a function primarily of aircraft weight, so if you're on a long-haul flight with lots of fuel on board then at the start your optimum may be quite low (obviously entirely dependent on aircraft type and engines etc, but I'll pluck some figures out of the air) -- say FL300 or thereabouts might be typical for a B744 or even lower. However, as the flight progresses and you burn off fuel, the optimum altitude increases -- so, for example, ten hours and a hundred tonnes or so lighter, the optimum might be around FL370. If you imagine it on a graph, it would be more or less a straight line between the two points.

 

In an ideal world we would just gradually climb as the flight progressed, slowly following the optimum level. Of course, in practice this isn't possible as it would make air traffic management a nightmare -- so we have to maintain an assigned level.

 

To ensure we can stay near the optimum level, though, once we've burned off enough fuel and the optimum altitude is now above us -- we can ask ATC for a climb to the next available level that is above the optimum. Once this is granted, we can climb to that level, level off, and repeat as necessary throughout the flight.

 

Most flight planning software (i.e. PFPX and Simbrief) will calculate the step climbs based on its performance profile for the aircraft in question. Obviously it knows your ZFW (because you told it before you computed the plan), it computes the fuel required for the route and knows how much fuel it's expecting you to have on board at any given moment, so it can use this information to calculate the step climbs (of course, it then becomes somewhat recursive because having climbed, your fuel burn will reduce accordingly from that point onwards, your ground speed will be different and so on).

 

However, your FMC will also calculate in "real time" the optimum and maximum altitudes. Because this is a calculation using "actual" sensed data at any given moment, it is unlikely to match exactly with the flight planner though it will generally not be massively different. Personally, I tend to go with the FMC's step climb calculation as it is using the real-time data: however, it would also be prudent to check the flight plan as it may be that there is a howling headwind 2000ft above and thus it may actually be less efficient to climb!

 

In terms of performing a step climb -- it's just the same as any other climb, really.

 

- Dial the new altitude in to the MCP

- ALT INTV (by pushing the MCP ALT knob in). This will set the new cruise altitude in the FMC and will also initiate a VNAV SPD climb.

 

You could dial the new altitude and select FLCH but then you would have to go in to the FMC and update the cruise altitude.

 

You could V/S it as well (and some prefer to do so as it is smoother for the pax) but it is not really recommended as you lose a lot of protections (if you select too high a V/S, the autopilot will continue to attempt to reach the selected V/S at the expense of airspeed. Result = stall). If you were to, however, the way I would do it is dial the new altitude in to the MCP, ALT INTV (to put it in the FMC) and then immediately hit V/S and adjust as needed.

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Thanks for the explanation. But after performing the step climb using the method you described above, will VNAV automatically go back to VNAV PTH on the FMA once the desired ALT is reached?

 

I use PFPX to plan my flights, and it gives me an initial flight altitutde, should I use the one given to me in PFPX or use the one that's assigned to me in the plane's FMC? I'm confused on this part.

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It should do, yes, provided the CRZ ALT in the FMC matches with the MCP (which it should if you ALT INTV).

 

I use PFPX to plan my flights, and it gives me an initial flight altitutde, should I use the one given to me in PFPX or use the one that's assigned to me in the plane's FMC? I'm confused on this part.

 

Depends!!

 

It doesn't really matter. In real life, the initial altitude on the flight plan will be the altitude that has been filed in the ATC flight plan, so that is what ATC will have on their flight strips as your requested cruise altitude and what they will clear you to (eventually). However, there is nothing whatsoever stopping you from getting on the radio and asking for higher if the FMC OPT is above the flightplanned initial cruise level. Doesn't mean to say you'll get it immediately (sometimes you may have been flightplanned at a lower level because of congestion higher up) but most of the time if the higher level is available you'll get it.

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