737NG flight Altitude Indicator

[framer]

At my outfit we set what we think the FL will be for the first hour or so at the preflight stage. Every time we change level after that we make sure the MCP, the FMC, and the Pressurization Controller all agree.
I see the logic in setting the higher altitude but have to say that I have never seen a problem with the way we do it. Is there an Engineering disadvantage to how we do it? Ie cycles of Pressurization weakening the fuselage?

[Skyjob]

Originally Posted by framer

Is there an Engineering disadvantage to how we do it? Ie cycles of Pressurization weakening the fuselage?

That is indeed why the recommendation is to set highest expected flight level pre-flight. The re-selection of the Altitude Selector in-flight can cause a change in maximum differential which is an unwanted event at cruise levels. This change in pressure differential can be prevented by selecting the highest expected cruise level pre-flight.

[Derfred]

There seems to be a bit of confusion here amongst different operators as to exactly what it is that Boeing recommends - and I'll start by rejecting Skyjob's assertions regarding pressurization cycles because there is simply no basis for it from Boeing. If there is (with respect Skyjob), please provide a reference.

First of all, when Boeing recommends (in the FCTM) setting the "final" cruise altitude, if you read the context in which this is written in the FCTM, what they are saying is do not set intermediate level-off altitudes that may be assigned by ATC during short term delays to reach your requested cruise altitude.

For example, if you have planned to cruise at FL370 (that is your top of climb - perhaps your flight plan suggests a step climb to FL390 in 3 hours' time - that is irrelevant), you would have FL370 set in the FMC and the pressurization panel. If ATC clear you to FL310, and advise to expect to be cleared higher soon after passing under crossing traffic, then FL310 is not your final cruise altitude so you would not set that in the pressurization panel. You would leave it at FL370. After a few minutes at FL310, ATC tell you to climb to FL350, FL370 will not be available due traffic. Now FL350 has become your final cruise altitude, so you would set that in both the FMC cruise page and also the pressurization panel. This is the point of the FCTM reference to "final" cruise altitude. If you happen to get lucky and get a higher altitude down track, then you would amend both FMC and the pressurization panel at that time.

So, "final" cruise altitude obviously means the altitude you finally expect to cruise at, at the completion of climb. It does not mean the altitude you expect to be at at top of descent. It also does not mean the highest expected cruise altitude for the entire flight (which may not even be the same thing - your flight plan may recommend a descent for wind reasons later down track - would you set that?). If that was what they wanted, then that would have been what they said.

There is no way anyone in my airline would cruise for 3 hours at FL370 with FL390 in the pressurization panel, purely on the basis that the flight plan suggested a step climb 3 hours into cruise (on ANY aircraft type, not just 737). And no (Skyjob), the cabin does not descend at 1000fpm when climbing from FL370 to FL390, it descends at around 400 fpm. I've been watching it do that for years. And also no (de facto), the descent procedure does not ask you to check that you have actually reached some imaginary highest cruise altitude that you happened to set in the pressurization panel some 5 hours ago, and all sorts of things have happened since, and you never got there... So getting an "Off Schedule Descent" warning is quite likely in the event you never made it to the altitude set in the pressurization panel, so off we go into a QRH checklist.

The FCOM pre-flight says "set cruise altitude". If they wanted you to set the highest expected cruise altitude for the flight, they would have written "set highest cruise altitude expected"... Unless you really believe they are that incompetent that they accidently left that bit out all these years.

Furthermore, the FCOM Supplementary procedure for Step Climb (and Step Descent) includes the following: "FLT ALT indicator... Set new level-off altitude". Why would it say that if the intention was anything other than to have the pressurization panel reflect the actual cruise altitude?

In summary, the reference to "final" cruise altitude in the FCTM can be mis-interpreted if you miss the context in which it is written. Obviously Boeing could have worded it better. It is a relatively recent addition which, if you go back and read it again, is obviously intended to address an inclination by some operators to set intermediate level-off altitudes in the pressurization panel - a practice that was never intended and they are trying to stamp it out. It is not a change of long standing policy which has always been to set intended top of climb cruise altitude. If you subsequently change cruise altitude, you change the pressurization panel, as you always did.

Lastly, to the best of my knowledge, this has been the same procedure for every pressurized airliner Boeing has ever built. Why would the 737NG be any different?

[Pin Head]

In good CRM terms, I kind of tend to disagree.

Final flight altitude is your expected final level in my opinion. If your at 390 for 40mins then so be it. Set 390.

That's how I read it and the English. I guess the lawyers would rad it his way too.

[Pin Head]

Investigation of pressurization incidents in the fleet has indicated that some operators are setting the FLT ALT before takeoff to an intermediate altitude and then resetting the FLT ALT one or more times during climb until the final cruise altitude or flight level is reached. Although this method of operation also provides a safe and comfortable cabin altitude for the duration of the flight, there are some considerations which may make this method of operation less desirable: • multiple adjustments of the FLT ALT unnecessarily increase crew workload during climb • intermediate FLT ALT settings are likely to result in higher cabin altitudes than if the final planned cruise flight altitude is selected. This is because the pressure schedule for lower altitudes uses lower differential pressure limits • if the flight crew does not reset the FLT ALT when appropriate, flying above the selected FLT ALT can result in an overpressure situation and activation of the pressure relief valves.

[de facto]

Derfred,
Agree to disagree.
I set the cruise(yes the highest) given on the flight plan,be it if any intermediate level offs.

For example, if you have planned to cruise at FL370 (that is your top of climb - perhaps your flight plan suggests a step climb to FL390 in 3 hours' time - that is irrelevant), you would have FL370 set in the FMC and the pressurization panel. If ATC clear you to FL310, and advise to expect to be cleared higher soon after passing under crossing traffic, then FL310 is not your final cruise altitude so you would not set that in the pressurization panel. You would leave it at FL370. After a few minutes at FL310, ATC tell you to climb to FL350, FL370 will not be available due traffic. Now FL350 has become your final cruise altitude, so you would set that in both the FMC cruise page and also the pressurization panel.

How to make easy complicated...I wouldnt,better things to do.
Again,an off schedule descent,if it occurs is a non event.(even if yes technically you need to read a single line in the qrh),however a flight above your "multiple reset" altitude is an event and possible incident if it goes as far as triggering the pressure relief valve.

[Pin Head]

I think we can all learn from this post.

A bit like the QRH and its recommendation not to troubleshoot, just do what the manuals says and then if you end up in a court of law then you can relax.

[Derfred]

It's not complicated. Whatever you set in the FMC cruise page, you also set in the FLT ALT.

For an intermediate level off, do you change the FMC cruise page? No. So you don't change the FLT ALT.

For a step climb or descent, you DO change the FMC cruise page. So you also change the FLT ALT.

Where is the manual does it say to set the highest expected cruise altitude?

[FlyingStone]

Actually it is safer to set the final expected flight level on the pressurization panel, since in this case you cannot forget to modify it during the climb. The only downside of this is that cabin altitude reach its final value at the beginning of the first cruise part instead of when reaching the final cruise level.

Take a look at this paragraph from FCOM:

Investigation of pressurization incidents in the fleet has indicated that some
operators are setting the FLT ALT before takeoff to an intermediate altitude and
then resetting the FLT ALT one or more times during climb until the final cruise
altitude or flight level is reached.

So, if we disregard the possibility that some operators used this as MCP ALT during climb (which I highly doubt), there clearly were pressurization incidents when using the "reset FLT ALT" technique during step climbs, so Boeing decided to strongly recommend setting final cruise level on the pressurization panel.

[Derfred]

So, if we disregard the possibility that some operators used this as MCP ALT during climb (which I highly doubt), there clearly were pressurization incidents when using the "reset FLT ALT" technique during step climbs, so Boeing decided to strongly recommend setting final cruise level on the pressurization panel.

They way they have written the explanatory background that you quoted indicates the opposite. They have specifically said that some operators are repeatedly resetting FLT ALT during CLIMB. Something which you highly doubt but it is annoying Boeing enough for them to issue an FCTM amendment.

They have not said that some operators are forgetting to reset FLT ALT during a step climb or step descent during CRUISE. Let me repeat that... They have not said there is any issue during cruise that they feel the need to address. If they had wanted you to set the "highest expected cruise altitude", don't you think they would have said that? I have not read the phrase "highest cruise altitude" in any Boeing document.

So, the important thing is not to confuse "intermediate altitudes during climb" with "step climbs during cruise". They are two different phases of flight.

The first is the CLIMB phase of flight. The second is the CRUISE phase of flight.

Boeing is clearly referring to the CLIMB phase of flight. That is why they use the phrase "during climb".

Intermediate level-off during CLIMB is handled with the MCP only. Step climbs and descents during CRUISE involve 3 settings: MCP, FMC and FLT ALT. I didn't make that up, it is SOP clearly published in the FCOM. If you can't find it, I'll give you the references.

Just set the FLT ALT to match the FMC CRZ ALT. Iif you subsequently change the FMC CRZ ALT, you change the FLT ALT. It's not complicated, and it's Boeing SOP.

If your operator publishes a different SOP then follow that. In that case the argument is with your operator not with you. But if your operator thinks they are following Boeing SOP, they may be mis-interpreting this FCTM amendment, and perhaps they should pick up the phone to Boeing and sort it out. Boeing will probably say "set final cruise alt", but get them to explain that. Final cruise alt is generally defined as TOC alt, not TOD alt (hence all this disagreement).

[Skyjob]

Originally Posted by Derfred

I didn't make that up, it is SOP clearly published in the FCOM. If you can't find it, I'll give you the references.

... It's not complicated, and it's Boeing SOP.

If your operator publishes a different SOP then follow that. In that case the argument is with your operator not with you. But if your operator thinks they are following Boeing SOP, they may be mis-interpreting this FCTM amendment, and perhaps they should pick up the phone to Boeing and sort it out. Boeing will probably say "set final cruise alt", but get them to explain that. Final cruise alt is generally defined as TOC alt, not TOD alt (hence all this disagreement).

I think there are different version of FCOM as each one is Operator adjusted, including yours, sorry to say. So your statements above refer to your operator interpretation of Boeing's SOP, the master versions can be referenced by your Boeing representative...

Let us just finish to say that although you are correct in the sense that you have a possibility of multiple CRZ levels, it is this very reason that Boeing refers to adjustments as one such climb may trigger the higher differential found (only) on NG when passing FL370. This very reason has caused issues, including high (yes up to 1000 fpm) cabin descent rates while the mode controller is adjusting to the new selected differential that is required. This very change in pressure during cruise is what is to be avoided, as per Boeing, and can be done by selecting final cruise level...

If your operator defines initial cruise as that to be set in Flight Altitude window, that is your SOP, other operators, especially those having experienced events as described, and those operating ETOPS with very long sectors (like those including me who flew a BBJ ETOPS 180) the final level set is that ultimately achieved for two reasons:

• passenger comfort during climbs, ths avoiding a high rate descend in cabin during climb at some point in flight;
• avoiding pressurisation changes during flight on the structure

[737aviator]

Skyjob, I think that Derfred has spent a great amount of time to comprehensively explain how and why most of us configure the pressurisation panel as we do.

I want to ask you a question though based on what you said before, as I just can't get my head around your logic.

Pre departure, we set FL390 on the pressurisation panel, as at some point 3 hours away its in our flight plan. Initial cruise is FL350 and we set this in the FMC. During the flight we step to FL370 and decide that the wind is good, 390 might be a bit bumpy, etc, maybe there's traffic resulting in 390 with a speed restriction, whatever, so we elect to remain at FL370 until TOD.

So we have 2 options;
1) We forget, as I pointed out before, and trigger an off scheduled descent master caution...Thereafter play with the flight altitude setting (action the QRH) and totally confuse the pressurisation controller.
2) We reset the pressurisation system to FL370, and therefore trigger a climb in cabin altitude as the differential pressure reduces, and this seems to be bad to have in your opinion.

Oh and that nicely leads on to the fact that you were cruising at FL370 for 3 hours with a differential pressure higher than what you should have had for flight at FL370...

[RAT 5]

If your operator defines initial cruise as that to be set in Flight Altitude window, that is your SOP, other operators, .........

After many years and many operators I am still bemused. I've seen many ways of flying the a/c. Generally they conform with basic a/c principles and manufacturer's recommendations, and thus acceptable. Operations of systems is very much the behest of the designers. Why would 'operators' profess to know better and publish SOP's of system operation that are not to manufacturer's philosophy? That has always been a point of suspicion to me. Sometimes it's been OK, and other times not a good idea. That it has been 'approved' has always been a mystery.

[Skyjob]

Originally Posted by 737aviator

1) We forget, as I pointed out before, and trigger an off scheduled descent master caution...Thereafter play with the flight altitude setting (action the QRH) and totally confuse the pressurisation controller.

An off schedule descend is a non-issue in most airlines, no big deal...

Originally Posted by 737aviator

2) We reset the pressurisation system to FL370, and therefore trigger a climb in cabin altitude as the differential pressure reduces, and this seems to be bad to have in your opinion.

Correct course of action, the pressurisation system has maintained a cabin altitude below maximum for the flight until now, resetting it to a lower level as further climb is no longer foreseen or planned by crew triggers the system to now use a lower pressure differential, causing the cabin to climb while aircraft is in level flight until the maximum pressure differential for the cabin has been reached.

From a technical point of view, in your option 2 all the pressurisation controller does is open the outflow valve a little bit and let some air escape to raise the cabin altitude.
Conversely when not selecting as described highest expected flight level pre flight, from a technical point of view, we would now be asking the aircraft to pump more air into the aircraft cabin at altitude to lower the cabin altitude inside the pressure vessel (your engines thus working harder due additional bleed demand) while trying to climb to the newly selected level at the same time.
Next time you try this, have a good look at climb performance when aircraft is adjusting cabin altitude and notice the difference when it has completed this task, you can see the difference in climb performance.

There are multiple ways to try and explain it to you, multiple reasons why Boeing is suggesting you do it as per advertised materials, including warning about flying above selected altitudes (as the controller is playing catchup to adjust to your newly created cruise level scenario which can be avoidable by following their guidelines), but please realise that unless you have flown the NG you are never and will never be confronted with this design anomaly of a pressurisation change due to differential changes in design when passing FL370...
If you choose to interpret the documentation of your manufacturer the way you do, please feel free to do so, hopefully you will not be a statistic one day when the system fails as the system is operating as advertised.
Meanwhile, please allow others including me to operate it more safely (according to Boeing) by selecting the highest expected flight cruise level pre flight and remove the risk to a more manageable one as no further action on the overhead panel is required...

As a side note, Boeing has placed by design, anything on the overhead panel which should be set pre flight. Unless directed to do so (eg by a checklist or changes in ambient conditions), no changed should be made to them. Just a thought