- a fairly common military procedure on combat aircraft, but remember that generally mil engines are 'optimised' for full power (leaving the Jaguar out of it :D) and so fuel consumption tended to be better at top end RPM, whereas civ and other transport a/c are 'optimised' for cruise RPM bands. Standard low fuel diversion in the BAC Lightning was one shut down if it was for any significant distance and we used to 'loiter' on CAP one-engined..

Whoever it was suggested staying at cruise altitude unpressurised - don't so it with me on board please. - where do you fly that has an offset procedure at 10,000' or below?

Highest ROD is achieved at max speed. You want to get down asap so use max speed unless a structural problem (i.e. structural damage) is known/suspected in which case most aircraft say to descend at current speed.

Engine out = longer range??
 

I'm fully aware of P-3's, Nimrods etc shutting some down when on patrol, but that's done for greater ENDURANCE, not RANGE. My question yesterday was whether range could be extended by this practice.

Potsie Weber surprised me with his 737 response - I guess I wasn't expecting a twin to behave that way (trim drag, y'know) but I now stand enlightened! :)

The genesis of my range question goes back 35 years: A F/E at KLM theorized the DC-10 was "hobbled" by the nose-down moment of the high #2 engine, causing excess stab trim drag. He proposed running #2 at reduced thrust in cruise to lessen trim drag.

He would have been correct, too, if it weren't for the unfortunate fact that engine SFC deteriorates with the reduced thrust, thus eating up any gains in trim drag. :sad:

Thanks @Potsie Weber!

What's the analog for a quad? Say a 747. Get's interesting ; would the adverse trim drag on a single engine failure be compensated by an additional symmetric engine shutdown. Only an academic question; doubt anyone's doing that.

Hi ross_M,
747-400 has flown LAX-LHR on 3 engines.
http://www.caa.co.uk/docs/33/factor200623.pdf

- see post #21? Endurance not toooo much of a problem on an emergency diversion:hmm:

Of course not. In case I didn't state it clearly enough, the planned speed I was referring to is the speed at which the flight planned time and range assumptions at the ETP for the contingency being considered are based. The actual situation at the time of occurrence may give cause to select a different speed. But since the fuel burn estimates used in the flight plan contingencies were calculated using a specific speed, it's a good place to start until the actual situation can be assessed and the plan adjusted accordingly.

Summarizing oceanic planning considerations into a format suitable for the purposes of a discussion like this is challenging to say the least. A comprehensive computer based flight plan package contains far too much information to whittle down to a few paragraphs. And there are more things to consider than can be accounted for in a few words. The automation of flight planning provides fast accurate data, but also makes understanding the assumptions and decision matrix more challenging. It all seems a bit more sensible when marked up on a plotting chart.

westhawk

Ha ha! Nowhere of course. The offset is intended to provide lateral separation from other traffic on the same track while descending.

There's at least an even chance you'll be changing course for a diversionary alternate or return anyway. I did not mean to imply that one must continue on the offset track once the descent is completed. If a real emergency, going direct might be most appropriate.

BA didn't quite make it to LHR. had to stop a bit earlier if i remember correctly.

"Question: Why does the descent have to be at max speed ? Why risk a structural overspeed instead of a low-speed stall ? Surely the RoD is more important to get down to 10,000' asap ?

e.g. A320 QRH Emer Descent : "Descend at the max appropriate speed."
e.g. B737NG FCTM Rapid Descent : "Target speed MMO/VMO".

Explanation appreciated. "


For the safety of the humans on board. rapid decompression at high altitudes can cause DCS (also known as the bends). Have to get down asap to get that nitrogen back into solution in the blood. plus, it might be a bit chilly.

whereas the DH 121 Trident had its centre-engine momentum drag above the c/l, and the thrust about in-line.
Slight assistance with trim drag :ok:

====

An historic miljet (1945 Gloster Meteor) regulary positions to airshows with one engine shut down. These are RR Derwents, some the earliest production Whittle based engines, and are stopped and started with complete confidence it seems, even in such an old warbird. They are a trifle juicy, so this makes a healthy improvement to its realistic range, as well as reducing hours on each engine (they both still have plenty left, fortunately :ok:)

D. P Davies pointed out that there are at least two ways of getting down fast.

The Mmo/Vmo route, anticipating the switch altitude to pickup Vmo.

Then it was possible to decelerate at constant altitude, hang out a few drag producing devices (e.g. u/c and/or spoilers ) and then come down slower & steeper.

For a veriety of reasons, the former is usually the best method, especially as it starts to get the cabin altitude down immediately....

Both methods (he suggested) required a sensible margin above level-off height (e.g. 10,000 ft) to initiate the levelling off in good time.

*The ear also doesn't like (can't handle) rapidly increasing pressure as much as reducing pressure. I believe I was once told that comfortable rapid descents should be limited to about 8000 ft/min... for that reason, though of course oxygen content of the cabin air would rate higher on the wish list, than some discomfort.

* Am quite prepared to be corrected on the above anecdote

Thanks for all your answers. I guess the max RoD is achieved at max speed (MMO?VMO) rather than min speed (Vls), although I have no reference for this. My A320 FCTM just says :

33 posts later and the question of the day is how to push the nose over and not exceed Vmo in a rapid decompression event.

How about some of the experts in here tell me how a 737 past the half way point to Hawaii, will either provide 150 passengers with O2 at altitude for the remainder of the trip or have enough fuel to fly the last 1100 Nm at 12500 feet.....

The explanation is based on the balance of forces in the dive.

At constant speed, the drag of the aircraft is balanced by the thrust and the gravity component along the flight path. The gravity component increases as the dive steepens, so for the steepest dive angle you want the most drag and the least thrust. Drag being proportional to speed squared, the faster you go the steeper the dive angle can be.

Furthermore, what counts is not dive angle but rate of descent. The faster you go for a given dive angle, the higher the ROD will be.

Therefore, for a given aircraft configuration, the faster you go, the higher the ROD will be, and its probably something like a cube factor on the speed. So speed is a very powerful determinant of ROD.

The only slight caveat is the statement "for a given aircraft configuration". Obviously if you change the configuration to achieve more drag, you can achieve a steeped angle. BUT if you give up speed limits in order to achieve a steeper angle, you may well (and in many cases do) lose out on ROD.

Going up the back side of the drag curve doesn't work because you get a fair amount of drag, but even if you could get the same dive angle as at Vmo, your rate of descent, which depends on both speed AND dive angle, will be anaemic compared to the VMO case - perhaps only 1/3 the ROD.

- heard of ETOPS?

You might break out of your FAR 91, small plane world and refer to FAR 121.333 and FAR 121.646. Airliners are REQUIRED to provide for a depressurization in passenger operations at the ETP. Wet footprints are NOT allowed.

I don't have a B737 FPCCM, but you can be assured that the plan has fuel for a diversion at the ETP, either onward to HI or return to CA. We do it in business jets, the airlines do it. Simple or you don't go.

GF

g.f.

I flew 121 quite often LAX-HNL and return. But, my experience was limited to TWA's L1011-100.

Westbound, once out of LAX about 300 miles SFO was our diversionary alternate until passing the ETP, at which point it became Hilo. Otherwise the wet footprint would be a problem.

Don't know whether that holds true to the 777 or A330 with their awesome range, but it seems like a B737-800 would be more critical than the L1011-100.

BTW, I am in Kauai this week. Landing at Lihue (PHLI) on Friday there was an Alaska Airlines 737 (800?, NG?) parked at the terminal. I presume it came from PANC. Or,perhaps Seattle? Anywhere else seems a bit of a stretch.

I think AS does PLHI-KSEA and PHLI-KLAX right now. -800SFP. All the islands are a stretch in a 737, really.