B737 emergency descent question

[downwind]

G'day guys,

Looking through the B737 QRH the other day and in the emergency descent checklist, the engine start switches are placed to "ON" in the procedure.

I thought they would be placed to "FLT" mode to offer more protection during the descent, to reduce the chance of flame out on the descent, why is this?

Do some airlines/pilots use "FLT" mode instead of "CONT" mode in this checklist??? (personal preference??)

It says in VOL 2 that;

"CONT position energises the seleted igniter(s) countinuusly".

"FLT position energises both igniters when the engine start lever is positioned to IDLE."

Does "FLT" assure the engines will not flame out on descent??

Thank You.

DW.:confused:

[Loony_Pilot]

Hi,

I presume you mean the Engine Start Switches (not the levers!)

There is really very little chance of flame out during an emergency descent.. its simply an idle thrust, at Max speed, with speedbrakes out descent.

The start switches go to continuous in the event of entering icing conditions during the descent, and yes they also provide additional flameout protection but as I said, its not any more likely to happen during a descent as any other time

[Astronomy Dominie]

If memory serves, the 737 had a bit of a 'history' of flameouts for a while (I think, with early CFM-56s, but excuse me if I'm wrong), and there are a few things about the way it's operated that hark back to that (the original problem relating to too low an idle in flight plus heavy rain - again, vague memories only).

Practically speaking, any time that I put the start switches ON for an emergency descent or heavy weather, they went straight to FLT, for just the reasons mentioned by downwind.

Must say that the flameout seems unlikely, but think about why you might be doing the emergency descent in the first place... If it's to do with a bleed-related problem, then you might want to be cautious.

[Ziggy]

Hi Downwind,

Our Memory drills also state ENGINE START SWITCHES - ON, wich could mean either to me, I guess.

Since a while ago though, we also have an expanded explanation in the Non Normal Maneuvers Section, stating to put the start switches to CONT. I don't have to agree off course but that's our SOP.

"Does "FLT" assure the engines will not flame out on descent??"

I don't think anything can ASSURE an engine won't flame out.

Greetings Z.

[Mr Levitator]

Here's my take on it........

Cont ignition is good enough for almost all situations. Boeing kindly thought that with the early problems with engines we had better have back up igniters. So they then put a switch so we could select either set to test them individually (on start up). However, if there was a flame out, it would be nice to have every thing available operating to get the engine relit. So as well as the CONT selection they put FLT in there to save us having to move a switch to the both position.

An emergency descent is no defferent than a normal VNAV PATH descent other than the higher ROD and MACH number. So CONT should do the job. I think the "Start Switches.......ON" means that as long as some form of ignition (cont or flt) is selected, the egines will be sufficiently protected.

A somewhat garbled reply, hope it makes sense.....
Mr L

[Flight Detent]

I agree, Mr levitator!

FD

[LEM]

Yes, ON just means ON.

Use, at your discretion, CONT in light or moderate icing conditions, and FLT in severe ones.

As usual.

[Astronomy Dominie]

Mr L wrote

An emergency descent is no defferent than a normal VNAV PATH descent other than the higher ROD and MACH number

Now just go carefully there! It's very different, in PATH, the aircraft follows a line in space, with speed and thrust changing to achieve it, whilst the ED is flown at constant selected speed.

[Mr Levitator]

Sorry, I meant VNAV SPEED (mach above FL250 and calibrated below).

But in a way, VNAV path is the same as an ED. VNAV is looking for a idle thrust descent to the touch down point. Usually however step heights stop this and require either a shallower gradient or a level off. But all the same, VNAV PATH in the ideal world gives an idle thrust descent.

VNAV also constantly adjusts the target speed in line with the cost index, so as in the ED, I guess the difference between LVLCH and VNAV is how the target speed is driven (your thumb or the FMC).

Prob wrong again though.........back to the newspaper

Mr L

[Flight Detent]

Both the takeoff and landing checklists specify ON as well!

:hmm:

[Lackof747]

Snecma (aka Smecma) did have some flameout problems on the CFM 56 (3B-2/3C-1 etc) engines in the earlier days. They were corrected by increasing flight idle RPM somewhat, as well as replacing the spinner cones with discs, thus dispersing rain water to the bypass of the engine instead of through the hot section.
This is what I´ve been told by three wise men.

Brgds,
Lackof747

[Terraplaneblues]

737 NG - in CONT posn. the engine is protected from flameout by the EEC, should it detect a problem (various criteria) both igniters would operate until the engine was running correctly, that is it would automatically be selecting FLT via the software.

737 Classic not as well protected - so selection of FLT would offer greater protection (dual igniters).

[Astronomy Dominie]

Mr L,

At the risk of a little thread drift, I fear you may still not quite have a grasp on this VNAV thing...

In VNAV speed, you'll get an idle thrust decent at whatever speed is programmed in the FMC, so you can programme a 250kts speed restriction at FL100, for example, and the aircraft will decelerate to meet it.

In VNAV path, an optimum path, that is, line in space (but definitely not a speed) is generated within the FMC. This path will change with CI ansd weight, but not 'constantly' unless you 'constantly' change the CI on the Perf Init page. The path's validity as the most economic way of getting back to terra firma also depends upon the accuracy of the forecast winds input to the FMC.

So, in path, the aircraft follows the line in space. THERE IS NO OVER-SPEED PROTECTION. If the path has been calculated to be steeper than the aircraft can manage at idle thrust, the speed will increase, right up until you go through the barber's pole. (You will get a message asking for drag). If the speed decreases significantly (I think I recall it's 20kts below path speed), thrust will be added. Very commonly, this has to do with encountering positive wind shear shortly after commencing descent.

The Mark 1 Human Brain has phenomenal processing capacity; the 737 FMC is a bit of an Amstrad amongst Dells these days. I've given up on VNAV, and now only ever descend in LVLCH or V/S, and I'm very happy with that!

[Lackof747]

On the 737-300/400/500 there is an overspeed protection in VNAV path, i.e. if drag is not added then it reverts to VNAV SPD descent (+15 knots if memory serves me correctly). Then the airspeed is the governing factor. I used VNAV SPD a lot when I flew the 737, because it remembers the 250 KTS speed restriction below FL 100, something LVL CHG does not thus demending me to be awake :E

Hope there is a drift correction on this thread!

[SR71]

Hmmmm....

Placard limit reversion can effect itself in slightly different ways depending on whether the AFDS and AT are in/out.

In a VNAV PTH descent the first FMC advisory message when speed is fast is at +10kts - DRAG REQUIRED. I normally ignore that one.

At +15kts you get an OVERSPEED DISCONNECT message and there is a mode reversion which, if Lackof747 is correct, is to VNAV SPD or LVL CHG. I thought it was the later - that might just be Update 10.2.

Similarly, if low on the programmed descent speed, the AT annunciation changes, I believe, to FMC SPD to acquire the correct speed again before returning to ARM.

The FMC Guide indicates that VNAV PTH is never supposed to exceed the barbers pole, but that in the time it takes to pitch up, the clackers may be set off.

It also suggests that in Update 10.2, 7kts prior to Vmo/Mmo, reversion to LVL CHG takes place.

I believe the important point is, however, that there is a degree of protection (albeit I would not rely on it myself - the time constants in some of the FMC feedback loops are far too large in my opinion!) built into a VNAV PTH descent in that it respects a placarded limit.

In this respect it is the same as a V/S descent where the thrust levers are on the idle stop. Our company SOP prohibits this, although in my opinion, accepting that a diligent pilot should always be observant of what the aircraft is doing, in principle there is no difference between such a form of descent and a VNAV PTH descent. Both will chase a particular trajectory through space at the expense of small deviations in airspeed. The only difference is a FMC advisory message or two.

I used VNAV SPD a lot when I flew the 737, because it remembers the 250 KTS speed restriction below FL 100, something LVL CHG does not thus demending me to be awake

At the risk of being a pedant, you're one of the guys I'd catch out then by removing the restriction surreptitiously when he wasn't looking! :E

In VNAV path, an optimum path, that is, line in space (but definitely not a speed) is generated within the FMC. This path will change with CI ansd weight, but not 'constantly' unless you 'constantly' change the CI on the Perf Init page.

Again, to be a pedant, the FMC uses cruise winds at cruise altitude linearly interpolated to zero wind at the destination to compute the path. The linear model can be made more accurate by entering winds on the DESCENT FORECAST page of the FMC although I'm still not sure of exactly how the modification works. However, any change to the route in the FMC, such as a simple speed change, a route modification, an altitude constraint will force a recomputation of the path using current winds. We see this everyday and it leaves us either high or low on the previously computed path depending on the circumstances.

I always try and use VNAV as much as possible, but like everything in the digital world:

Garbage in, garbage out

:ok: