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MED1473

Correct management of cruise FL change and constraints

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Hi all!

I was wondering if anybody can enlighten me on the correct management of cruise FL changes. I'm familiar with the STEP CLIMB procedure to adopt a higher, more economical FL as fuel is burnt during cruise bit what about a LOWER FL? What is the correct, most professiona way of doing so?

I'm refering to FL constraints at WPs that are often encountered (especially in Europe).

As an example I often fly from EGNM or EGSS to LIMF and although the optimal FL is always around FL370+ there is on the route (wp XAMAB) a FL constraint for flights directed to LIMF. Unless a massive diversion is done I am therefore required to cross XAMAB at FL350. But XAMAB is around 150+ NM from the TOD.

 

How should I manage this? In my view the otions are three and I'm curious to know which one is the more correct, professional one

1. Set cruise at eg. FL390 the insert an altitude constraint of FL350 at wp XAMAB (bit in this case the plane goes in DESC mode when descending to the new cruise level)

 

2. Set the constrained FL (FL350) as cruise FL and then step climb to the required FL (eg.390) and the descend again

 

3. Set the required FL (eg. FL 390) as cruise FL and the change the cruise FL to the constrained one (eg. FL350) calculating when to start the descent manually

 

4. None of the above

 

Thank guys

Matteo Capocefalo MED1473

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I have no doubt you'll get plenty of answers!!!...so I'll kick things off with my personal preference... in the NGX...

 

I'd be steering well clear of the LEGs page & doing everything via the MCP & the CRZ Page within the CDU.

 

For the descent I'd change the MCP ALT, type the new lower CRZ ALT into the SCRATCHPAD & then select it up to LSK 1L in the CRZ Page of the CDU & EXEC -

 

The aircraft would then commence a DES to the new lower CRZ ALT at approximately 1000fpm in VNAV.

 

If whilst in VNAV the rate of descent exceeded 2000fpm with approx 2000ft to go to the new ALT I would switch to V/S set at 2000fpm & passing 1000ft to the new ALT I would reduce this to 1000fpm to avoid a level bust & uneccesary TCAS alerts.

 

Once at the new CRZ ALT I would reselect VNAV - carefully checking the FMAs throughout the process.

 

Obviously if VNAV was performing well & not exceeding the 2000 & 1000fpm gates as the aircraft closes to within 2000 & 1000fpm of the new CRZ ALT I would let it control the whole process.

 

For the climb I would effectively do the reverse...if a new higher CRZ ALT is set in the MCP it is automatically placed into the SCRATCHPAD on the CDU for selection into LSK1L on the CRZ Page. Once again I would commence the climb to the higher CRZ ALT in VNAV.

 

I would switch to V/S if the vertical speed (in VNAV) exceeded 2000fpm with 2000ft to go to the new higher CRZ ALT & lower this to 1000fpm with 1000fpm to go for the same reasons I highlighted above.

 

Once at the new CRZ ALT I would reselect VNAV - carefully checking the FMAs throughout the process.

 

Obviously if VNAV was performing well & not exceeding the 2000 & 1000fpm gates as the aircraft closes to within 2000 & 1000fpm of the new CRZ ALT I would let it control the whole process.

 

A word of caution about V/S - unless you use it judiciously & carefully monitor the aircraft performance on the PFD it can hurt you as it provides no speed protection (think decaying IAS with increasing altitude = stall) & it will also fly you away from an MCP ALT under certain circumstances.

 

VNAV however provides speed protection & will always attempt to keep you between the low & high speed buffet margins on the SPD Tape.

 

If the aircraft was in bumpy, turbulent air I would ALWAYS avoid V/S altogether & let VNAV control both the descent & climb.

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Thank you Steve for your very exhaustive and useful reply.

Just one doubt remains really, managing the whole thing through the cruise page do I therefore need to calculate manually when to execute it assuming a 1000fpm descent or is there another way to ensure reaching a certain wp at a certain FL?

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No probs...

 

For the DES @ 1000fpm in VNAV & a CRZ Level change of say 4000ft - obviously that will take 4 minutes - commence the DES by the aircraft clock when there are 4 minutes left to run to run until the ETA for the WPT in question.

 

Climbs can be slightly more tricky & you might arrive slightly earlier at the new CRZ ALT but you always have the option to 'wind away' some V/S in V/S mode - obviously keeping a close lookout on your PFD as you should really monitor aircraft performance carefully in V/S mode especially at the higher altitudes.

 

For a STEP of 4000ft to a new CRZ ALT I would estimate commencing the CLB once again at about 4 minutes to run to run until the ETA for the WPT in question,

 

If your preference is online flight ATC might help & give explicit instruction including the use of best rate of climb if they needed it - in this instance I'd rely solely on VNAV to do the work for you safely & efficiently.

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