737 VNAV Level Off during Descent
 

Guys, today I had the pleasure of jumpseating in a company 737.

I witnessed something during the descent which confused me slightly. I didn't want to ask the guys as they were very busy and sterile cockpit etc.

They were told to descend FL240 level at KRISE.
They had KRISE /FL240 in the FMC.
Shortly later they were told to descend FL190.

As soon as they reached KRISE, the aircraft levelled off at FL240 and I caught this new green TOD appear on the ND. They realized they levelled and went V/S.

This sort of caught them by surprise and even myself.

Surely after KRISE, where the a/c had reached it's altitude restriction, and with 190 in the MCP ALT, the a/c ahould have continued it's descent?

Do you remember what the FMAs were?

Our 737s do the same now - the geometric path descent feature of the FMC has been removed due to some issues and this is now the default behaviour on our fleet too.

Effectively, the FMC calculates a new ToD after a constraint and the aircraft will level off. We usually intervene with VS until the VNAV path is captured again.

Prior to the approach environment (where the behavior changes*) VNAV will always fly level until it reaches an idle path to the next relevant restriction. With multiple restrictions, it will stair-step down this way, between level and idle. The MCP altitude is irrelevant to all this, except that if it happens to be reached at any point in the process, it will level there (annunciating VNAV ALT instead of ALT HOLD)

So after KRISE at 240, it is normal for it to level until the next top of descent (even if there is nothing prior to the discontinuity, and this next top of descent doesn't exist). If you want to proceed down early, you have to specifically tell it somehow.

The "somehow" is where you start getting differences between software revisions, customer options, etc. V/S is always one way, of course, which completely disregards your positioning in relation to any future fixes. On ours, there is the ability to stay in VNAV, push the ALT INTV button on the MCP (or select DESCENT NOW from the descent page in the FMS, which does the same thing except in one somewhat convoluted edge case) and it would take the 190 from the MCP, set that as the new VNAV level altitude, and begin a 1000 fpm descent (annunciating VNAV PTH) to get there.

* on an approach, instead of stair-stepping down at idle, it draws a slope between the altitudes at each fix.

The aircraft behaved exactly as I would have expected it to based on your description. Bit worrying that it confused your 737 pilots, maybe time for them to get back into the FCOM and FCTM and read up about VNAV?

Thank you gents, all crystal clear now!

No panic or sweat from the lads at the pointy end. We save that for the sim!

The 737 is still new to our fleet and I have been offered a transfer onto it. I am yet to decide as I do enjoy the "flying tray table" right now!

Just a small guess: "I have flown nothing but 737NG in one company" applies to you?

I can give you multiple reasons why it would surprise even very experienced people. They saw it, they corrected. Job well done.

And sitting on the 737 jump seat did not make the decision a no-brainer? Would be a sad transfer. But to each his own I guess…

If one is sat on a 737 jumpseat one’s brain is surely just full of thoughts of searing back pain…. :E

I’ve flown both, and will take the A320 every single day of the week. The 737 is a lot of fun, but so is my BSA, and I ain’t riding that every day either.

What does what I fly have to do with the fact this crew doesn't seem to understand the basics of aircraft they fly?

What was described there should not even surprise inexperienced pilots on type.

I have flown several Boeing types and when I changed to the 737 this behaviour confused me too. The VNAV error bar can also be misleading under these circumstances.

The behaviour I find more concerning though, is when altitude intervention is pressed before TOD and the aircraft commences descent at 1000fpm before intercepting the path from below, the FMA is VNAV PATH. Other Boeing aircraft would annunciate VNAV SPD. It is not on the VNAV path, it is below it. I find this FMA misleading.

That’s not to say I don’t enjoy flying it, mind you.

It is not the idle path ("the" path) but it is a path.

Well, we don't know if, once computed, it is exactly a path (in which case, if the wind changes, the descent rate will change a bit from 1000 fpm) or it is exactly a 1000fpm descent rate (in which case, if the wind changes, the path slightly changes). But in either case, it is either exactly, or almost, a path; it is definitely not a speed. The main pitch/thrust interaction slots in the the overall path philosophy: the pitch channel is flying a path (or descent rate) and the thrust channel is maintaining speed.

No offence meant but a 737 and a 737 are not the same. Technical knowledge is important but it quickly creates a false sense of safety as well. Safety is much more about realizing you might not know. Unexpected behaviour is part of the job and dealing with these situations part of our job.

Since you refer to FCTM I gladly refer to the chapters on automatic flight where Boeing explains ”deviations from expected performance are normally due to an incomplete understanding of their operations by flight crew” and how the pilot should reduce the level of automation until... it literally discusses unexpected level offs etc. You just state that incomplete understanding is unacceptable, however Boeing says it is perfectly possible. The crew acted according to the FCTM you referred to.

There’s the bunch of pilots who’ve only flown 1 aircraft in 1 company and think this is the world to be known from day 1 by every pilot on their fleet. And there’s the bunch of pilots who have seen other Boeings and fleets and realise it can be pretty confusing. But they understand the philosophy of monitoring and making interventions if necessary.

ps: when became VNAV ALT an option on the 737?

For anybody curious, a quick update.
Just before reaching the waypoint with an altitude restriction, you can press ALT INTERVENTION button on the MCP and it will allow the aircraft to continue descending. It just places the aircraft into the default 1000fpm descent until it recaptures the new VNAV calculated path.

I am still trying to read through some FCOMs to find out if there are any requirements for this method to work.

Every time you press the button it deletes the next constraint. Going up or coming down. Quite useful and going back to a 'non' model you really notice how much heads down time it saves. That and not getting kicked out of vnav at every level off.

Yes but if youre climbing to a level above crz alt, pressing it will set the new crz alt?
If you descend (let's say FL390 to 370), does pressing it enter a new crz alt or will it fill the scratchpad with 37000?

1.Yes it will set new CRZ ALT
2. Depending on distance from TOD. More than 50 nm - it will set new CRZ ALT. Less than 50- will enter early descend (1000 fpm till intercept idle descend PTH).

Of course it’s about my 737 that I operate or operated.

In my previous company only some of our 737 NG had VNAV ALT, others had geometric path.

On "our" 737s, I'm not quite sure what has happened, but pressing ALT INTV no longer gets the aircraft descending after it has levelled off at an assigned restriction, which is bizarre because when you press it before the initial T/D at cruise level, it does start to descend (at 1000fpm until it intercepts the 3 deg path).

e.g. a restriction of FL150 by KIDLI. Before KIDLI we are further cleared to FL110. Setting FL110 in the MCP causes a new "T/D FL150" to appear a few NM after KIDLI. Pressing ALT INTV when the aircraft is about to, or has already levelled off at FL150 for KIDLI does not keep VNAV coming down. I can bet the house that when I started flying the 737 over 2 yrs ago pressing ALT INTV did the exact same as above, it would again descend at 1000fpm until it intercepted the 3 deg path leading to FL110 - but it doesn't anymore.

Regarding the crew in the OP's scenario, it sounds to me like they just got distracted, forgot, or for whatever reason didn't notice at the time that it was the previous restriction was there. I don't think it offers any insight into their competency on the type whatsoever.

As for the VNAV on the MAX, the less said the better. In VNAV PTH we once had an early descent, the aircraft began descending, and then started a climb! We went V/S to fix it, we never quite got to the bottom of it. From what we could make out, the aircraft had a moment where it was attempting to intercept the VNAV PTH which was above us. Between that and the yoyo'ing of 9999 low up 9999 high and every number in between it's a bit of a wild one at times.