When to initiate Step Climbs?

[AirbusCG 52]

Good morning/afternone/evening/night everyone,

do we have any real 747 pilots in here to shed light into the question when to start a step climb in the 747-400/-8?

In my airline it is SOP to step up only once the OPT FL is equal to the step level to avoid getting too close to coffin corner.
Looking at most altitudes flown by actual 747's on FlightRadar it seems that most airlines actually do the same (judging from the altitudes flown vs what PMDG gives me as OPT using the maximum realistic ZFW for such flights, aka full pax load plus a few tons cargo). But then again in the PMDG 747 the coffin corner does by far not get as close as it does on my real aircraft when flying 1000ft above the OPT level, so I see nothing that would really warrant using my airlines SOP on a 747.

That's why I started to wonder what procedures are used in real 747's. Do you just follow the FMC step climb suggestions? Or rather your OFP? Or something completly different?

Edited September 25, 2018 by Emi

[scandinavian13 5,192]

Just now, Emi said:

do we have any real 747 pilots in here to shed light into the question when to start a step climb in the 747-400/-8?

When it says STEP: NOW, and ATC approves it.

1 minute ago, Emi said:

In my airline it is SOP to step up only once the OPT FL is equal to the step level to avoid getting too close to coffin corner.

I'd argue that this is a bit overly cautious, but you're the captain of the ship...

1 minute ago, Emi said:

Do you just follow the FMC step climb suggestions?

I do.

1 minute ago, Emi said:

Or rather your OFP?

They'll usually be about the same. Keep in mind that the OFP will specifically attach the new altitude to a waypoint in the plan, and not the more optimal actual step point (i.e. it will assign the higher altitude to an existing point, rather than create a pseudowaypoint in the middle of the flight plan to show the climb). Additionally, as good as our data collection is now, keep in mind that the wind aloft data is still listed as a forecast product, because it's really not as accurate as it should be, or updated as often, to be considered hard, actual, data. That in mind, the OFP's data is only really a work of planning, and only really relevant to give you an idea before you launch (though it does give you a good idea of data tracking as you fly to be sure burn numbers are okay).

[AirbusCG 52]

Hi Kyle,

thanks for your quick reply!

I agree it may sound a bit overly cautious on the first thought, but seeing that 1000ft above the OPT we're usually just a few hundret feet below the max (at least in my aircraft that is) it actually makes sense if you do not constantly want to have a hand at the speed brake lever in case even the slightest turbulence kicks in.

I guess on the 747 it's not that much of a problem since the max is usually still some 2000ft or higher above the OPT:
Yet looing at many 747 flights on FlightRadar they are usually some 2000ft lower than what I'd get to if I follow the FMC step climb suggestions.
I konw I know, you can't compare without having the actual numbers, but the sheer number of flights flying so much lower just makes me think there must be something going on in the 747 which I am unaware of.

I share your thoughts on the OFP, on the other hands side though I see it quite often in real life that the OFP FL (LIDO based sytem we use) differs quite a bit from the FMC. Now obviously LIDO takes a lot more factors into account than the FMC does, no doubt about it. It might just be our lovely european airspace screwing it over 😉
I'm really impressed by the wind accuracy anytime I see though. It's usually within +-5kt and degrees which is quite amazing seeing that most flights are planned some 12 hours before the actual flight.

[scandinavian13 5,192]

3 minutes ago, Emi said:

I agree it may sound a bit overly cautious on the first thought, but seeing that 1000ft above the OPT we're usually just a few hundret feet below the max (at least in my aircraft that is) it actually makes sense if you do not constantly want to have a hand at the speed brake lever in case even the slightest turbulence kicks in.

Not quite sure of the assertion here. The cautionary band is a cautionary band, not a limit. If there's turbulence, then take a different approach. I don't see a need to set all ops on an assumption that there will be problems. That kind of blanket approach is a bit like taking full fuel everywhere, in my opinion.

5 minutes ago, Emi said:

I guess on the 747 it's not that much of a problem since the max is usually still some 2000ft or higher above the OPT:
Yet looing at many 747 flights on FlightRadar they are usually some 2000ft lower than what I'd get to if I follow the FMC step climb suggestions.
I konw I know, you can't compare without having the actual numbers, but the sheer number of flights flying so much lower just makes me think there must be something going on in the 747 which I am unaware of.

You definitely need the actual numbers, and you also need to take a look at flow. Last weekend, we were climbing out and I heard "climb and maintain FL220 - that'll be your final." ATC restrictions come into play, too.

7 minutes ago, Emi said:

It might just be our lovely european airspace screwing it over 😉

Very likely. The altitudes there can be very odd. Makes sense in a larger picture, but some of the plans I've seen have been full of all kinds of altitude restrictions.

[AirbusCG 52]

5 minutes ago, scandinavian13 said:

Not quite sure of the assertion here. The cautionary band is a cautionary band, not a limit. If there's turbulence, then take a different approach. I don't see a need to set all ops on an assumption that there will be problems. That kind of blanket approach is a bit like taking full fuel everywhere, in my opinion.

Depends on what you classify as getting close to the limit. We're getting to about 10kt below Mmo at 1000ft above the OPT, for the guys in the HQ making out SOPs that is close enough to want us to fly lower.
With turbuelnce you want greater margin anyway, no doubt about it!

Quote

You definitely need the actual numbers, and you also need to take a look at flow. Last weekend, we were climbing out and I heard "climb and maintain FL220 - that'll be your final." ATC restrictions come into play, too.

Looking just at particular flights I certainly agree with out, no way to tell anything without numbers. That's why I wrote I got that impression by just looking at the sheer number of flights over the last couple of months where the majority just cruises lower.
Sure enough, not sufficient data to make any certain statements, enough to get wondering and asking though 🙂

Quote

ery likely. The altitudes there can be very odd. Makes sense in a larger picture, but some of the plans I've seen have been full of all kinds of altitude restrictions.

Fully agreed, those EU planning constraints can be a pain in the a**. Luckily most of the time we still get out prefered cruise level though!

[PGBosak 4]

2 hours ago, Emi said:

In my airline it is SOP to step up only once the OPT FL is equal to the step level to avoid getting too close to coffin corner.

From my understanding of aerodynamics you/your virtual airline have/has a wrong perception on the principle of coffin corner.

This phenomenon you're taking into account appears beyond service-ceiling solely and this service-ceiling is being constant, based on test-flights with security margin. So your climb performance within your legitimate planning stepclimb-range has nothing to do with taking risk in relation to coffin corner, because if you really get near to that phenonemom you are already beyond service ceiling. That shouldn't ever happen.

Coffin corner shows a situation where stall speed and critical Mach number gets equal. So you have to fly higher than FL451 with 744 or higher than FL431 with 7474-8 to experience the risk of coffin-corner.

Edited September 25, 2018 by PGBosak

[AirbusCG 52]

11 minutes ago, PGBosak said:

From my understanding of aerodynamics you/your virtual airline have/has a wrong perception on the principle of coffin corner.

This phenomenon you're taking into account appears beyond service-ceiling solely and this service-ceiling is being constant, based on test-flights with security margin. So your climb performance within your legitimate planning stepclimb-range has nothing to do with taking risk in relation to coffin corner, because if you really get near to that phenonemom you are already beyond service ceiling. That shouldn't ever happen.

Coffin corner shows a situation where stall speed and critical Mach number gets equal. So you have to fly higher than FL451 with 744 or higher than FL431 with 7474-8 to experience the risk of coffin-corner.

Hi Paul,

Coffin Corner will also appear at lower altitudes than service ceiling if the aircraft is heavy enough.

In simple terms it looks like this: Higher weight -> More lift required -> higher AoA -> Faster airflow over wing -> earlier shock induced seperation on the wing due to shockwaves.
You can easily try this yourself: Simply load the 747 fully up, set full thrust and keep it climbing. After a short while you will see high- and low speed limits getting closer and closer until they are equal. That's coffin corner and at full load it will be reached much earlier than service ceiling.

The critical Mach number has nothing to do with Coffin Corner as it is constant for a certain wing profile and does not change with weight.
For a 737 for example, if I'm not mistaken, it is M.615.

 

[PGBosak 4]

41 minutes ago, Emi said:

Hi Paul,

Coffin Corner will also appear at lower altitudes than service ceiling if the aircraft is heavy enough.

 

Hi Emi,

several years ago i borrowed a book about aviation and it stated that coffin corner is one of the parameters that sets the limit for service ceiling. About the correlation between stall speed and critical mach number in relation to coffin corner you can even read in Wikipedia. 

 

A real pilot answers the question: "What is coffin corner?" in that way:

Answer: Coffin corner is a term used to describe a condition at high altitude when the maximum speed (limited by the spreading of supersonic shock waves) and the minimum (limited by amount of air passing over the wing) are nearly the same. This has caused cases where the airplane could not fly faster due to the high-speed limit or much slower due to the low-speed limit, making it difficult to control during turbulence or when descending.

One extreme example of the coffin corner is the U-2 – the difference between the famed spy plane’s high-speed limit vs. low-speed limits is quite narrow at the extreme altitudes of 60,000 feet or more. It is a very challenging airplane to fly because of the small acceptable airspeed window.

Most modern jetliners have good speed margins, making coffin corner problems an issue of the past.

source:

https://eu.usatoday.com/story/travel/columnist/cox/2015/07/24/coffin-corner-cruising-altitude/30615399/

as you are german, the german wikipedia-site also states:

"Die Dienstgipfelhöhe liegt immer unterhalb des Coffin Corner, da sie dadurch definiert ist, dass noch eine Steigrate von 100 ft pro Minute beim Propeller-Flugzeug und 500 ft pro Minute beim Strahlflugzeug möglich sein muss. Dies wäre jedoch in der Nähe der Coffin Corner nicht mehr der Fall."

another source:

https://www.skybrary.aero/index.php/Coffin_Corner

"In commercial and general aviation operations, flight at altitudes approaching coffin corner is generally avoided. Compliance with the aircraft manufacturer's maximum operating altitude in ISA conditions given in the AFM, if necessary taking account of the difference between actual conditions and those of the ISA, will normally ensure that adequate buffet margins are maintained for both the high and low speed boundaries."

 

Are all of those sources wrong? 😉

 

 

Edited September 25, 2018 by PGBosak

[AirbusCG 52]

Hi Paul,

I really apprechiate your research! Your resources are, apart from Wikipedias definition of coffin corner, not wrong, however easily misinterpreted.

Wikipedia says the following:

Quote

Coffin corner [...] is the region of flight where a fast fixed-wing aircraft's stall speed is near the critical Mach number, at a given gross weight and G-force loading.

What they mean is the maximum mach number, not the critical macht number. Those are two different things, critical mach number means the mach number where locally on one point of the aircraft supersonic speed is reached, usually on the top of the wing. Modern jets cruise far above this speed however. On the 737NG for example the critical mach number is .615, cruise speeds are around .76-.79 though.
All aircraft cruising in the transonic range have a Mach Trimmer installed to compensate for the effects when passing the critical mach number and therefore safe flight is possible above Mcrit.
I'd love to give you some quotes here, but I'm afraid all my ATPL theory books are in German language only.

Quote

several years ago i borrowed a book about aviation and it stated that coffin corner is one of parameters that sets the limit for service ceiling.

That's right, however the fact that coffin corner can limit service ceiling does not mean that service ceiling can limit coffin corner. Keep in mind that the service ceiling is not defined for an aircraft at the maximum takeoff weight.
A fully loaded 747 will for instance by far not be able to reach its service ceiling. Yet you can encounter coffin corner regardless of the weight of the aircraft, but the heavier the aircraft, the earlier you will encounter it.

Quote

Most modern jetliners have good speed margins, making coffin corner problems an issue of the past.

Depends on what he means with modern jetliners, from what I have seen in flight simulation the 787 and 747-8 indeed have some very good margin, so do most Airbus aircraft.
On some Boeing types things look a bit different though, take for example the NGX, fly it up to the Opt+1000ft and you'll be only a few hundret feet below the maximum.
Now fly a 30° angle of bank and take a look at your speed tape. Margins to both sides will become quite low there! The top of the amber band provides overshoot protection until 45° angle of bank to stick shaker, which in return again has a small margin to actual stall. So at the FMC MAX you are just about okay. But if you take it any further...
If you do this at MTOW with the -900 you will see this effect long before you fly at the service ceiling of FL410.
Some newer FMC revisions will even indicate whether the max altitude is limited by thrust or buffet margin. Unfortunately this is not the case with PMDGs FMC revision.

Anyway, we start drifting offtopic here 😄
 

[PGBosak 4]

For a moment I wondered if I could have mixed up ceilings or interpreted them wrong. Then i've read this paragraph:

"The point where high-speed Mach, IAS, and low-speed buffet boundary IAS merge is the airplane's absolute or aerodynamic ceiling. Once an aircraft has reached its aerodynamic ceiling, which is higher than the altitude limit stipulated in the AFM, the aircraft can neither be made to go faster without activating the design stick puller at Mach limit nor can it be made to go slower without activating the stick shaker or pusher. This critical area of the aircraft's flight envelope is known as coffin corner."

[FAA Handbook FAA-H-8083-3A, page 224]

taken from: 

http://code7700.com/aero_coffin_corner.htm

That precisely states coffin corner and also aerodynamic ceiling don't "drift down" to any aircraft altitude state within normal operating FL. It's a theoretical issue nowadays. There are tons of visual graphics and expertise on the internet which insist it does, starting with wikipedia. From my perspective they must be wrong if FAA is being right. This phenomenon appears not only above service ceiling but also beyond absolute ceiling. 

And the low climb rate which defines the service ceiling will prevent you to ever get near coffin corner in real..

I know we are off-topic but i find it interesting to debate, because there are a lot of reverse assumptions out there. You wouldn't have to define those ceilings if coffin corner may appear at any upper operational flight-level. Doesn't make really sense. 😉 Those certifications are here to avoid any risk in relation to this topic.

cheers

 

 

 

 

 

Edited September 25, 2018 by PGBosak

[berts 97]

In practical terms if you stick to the FMC predictions you can't go wrong! 

In the early days of the 747 the airframe outperformed the engines, but today's engines can easily outperform the wing and you can get into serious trouble if you climb too early.  It is possible to see the red min and max manoeuvering tapes come close together, especially in turbulence or a prolonged turn when close to the maximum altitude for the gross weight.

It is all a question of judgement on the day, but if you do climb early make sure it is in smooth air and you are not going to have to descend again for many hours before TOD.

[skelsey 765]

10 hours ago, Emi said:

That's why I started to wonder what procedures are used in real 747's. Do you just follow the FMC step climb suggestions? Or rather your OFP? Or something completly different?

The guidance I have seen is increasingly to follow the OFP on the basis that the levels will have been highly optimised based on the winds, whereas the FMC is often a bit more of a blunt instrument and may not necessarily take the fact there there's a nice jetstream at FL330 giving you a better speed over the ground in to account when it asks you to climb to FL350. The other issue (at least in Europe) is that a route may be either level-capped (by RAD) or to avoid CFMU slot restrictions -- many sectors are split by level and so planning a lower level may avoid a restriction put in place to prevent the higher sector from being overloaded and thus prevent a long takeoff slot delay.

[thibodba57 462]

1 hour ago, skelsey said:

The guidance I have seen is increasingly to follow the OFP on the basis that the levels will have been highly optimised based on the winds, whereas the FMC is often a bit more of a blunt instrument and may not necessarily take the fact there there's a nice jetstream at FL330 giving you a better speed over the ground in to account when it asks you to climb to FL350. The other issue (at least in Europe) is that a route may be either level-capped (by RAD) or to avoid CFMU slot restrictions -- many sectors are split by level and so planning a lower level may avoid a restriction put in place to prevent the higher sector from being overloaded and thus prevent a long takeoff slot delay.

This is a bit different now with the Recommended Altitude on your VNAV Cruise page (NG FMC).  Our guidance now to disregard the OFP steps and climb when the CDU recommends a step.  Even then you have to do your own look ahead.  The Recommend is only looking 500NM ahead it might tell you to go back down an hour later.  At that point it might be a wash in the climb burn really.

Edited September 26, 2018 by thibodba57