Engine Shutdown - How Long Before Safe?

[BugSmasher1960]

In the case of something like a 777 or 787 - does anyone know how long one has to wait from an engine shutdown being initiated to when it's safe for passengers to be evacuated in the vicinity/in-front of it?

[NSEU]

It doesn't take long for the thrust to disappear. The engine "fire" goes out almost immediately. As a ground maintenance engineer, I used to put safety cones a meter or so in front of engines shortly after engine shutdown (sometimes, I'd say, as quickly as 30 seconds). The noise of the engine blades spooling down was a little intimidating though.

I'd leave the cones at the back of the engine a little longer, but that's only because I didn't like burnt fuel fumes :yuk: Sometimes cargo (un) loaders would approach the aircraft before the engineer at the nosewheel gave the thumbs up, but they didn't seem to suffer any ill-effects.

Perhaps the pilots can comment on what %rpm they turn off the red beacon/strobe light?

[BugSmasher1960]

Thanks NSEU - I'm kinda after a figure though. I'm guessing that even without ignition, you still have a heavy mass of fan blades pulling some air in - would just like to know at what point that becomes insignificant.

[FullWings]

I’d say that if you’re actioning the evacuation checklist, by the time the slides have deployed and people are arriving at the bottom of them, there won’t be a danger from the engines unless you climb into the inlet and stick body parts into the fan.

Our Shutdown Procedure has the beacon going off last and if you do the procedure in a non-rushed manner, it’s going to be 5-10s between fuel control switches and the beacon. Even on a GE90-115, I don’t think there’s a danger of ingesting anyone that walks past the front at that point.

[JammedStab]

Back when I was a baggage handler, the official waiting time(according to our company) before approaching a CFM engine on a 737 after shutdown was 30 seconds. I am sure that there is some extra time added on to when the risk actually has passed and you won't get sucked in at 29 seconds but there was a waiting time.

Of course, that is in front of the engine.

[Metro man]

A320 beacon off at 20% N2. Park brake not released before this either because of residual thrust even with the chocks in place.

[Fursty Ferret]

Almost instantly (assuming idle or near idle speeds at shutdown). Engines don't like doing work with no fuel and just the act of turning the compressors as it spools down will ensure that any ability of the fan to force significant quantities of air through the engine will be gone within seconds.

Don't forget that the fan is essentially a so.lid disc without much momentum due to its light weight. You're then asking it to do work as it decelerates anyway

*caveat - I'm not prepared to test this theory personally.

[lomapaseo]

What we need is data like a balloon held in front of the inlet after about 4 secs and more after the fuel is stopped.

More likely just casual wind blowing loose objects (clothes, sunglasses, etc.) into the fan and spit back (seen that happen).

[Valmont]

A320 Airbus SOPS: Cabin crews disarms the slides as soon as the engines are shut down, you then wait for the slides to be disarmed to put the seatbelt sign off and the beacon off. It will be below 5%N1 on CFM engines when you put the beacon off.

[Derfred]

737 Normal ops: Beacon off at 15% N2. Idle N2 is above 40%. Takes at least 20 seconds to reduce to 15%.

I'm sure that's very conservative for ingestion hazard, however. Need the Mythbusters to set up Buster in front of an engine one day...

[BugSmasher1960]

Thanks folks.

Looks like somewhere between "instantly" and "at least 20 seconds" :confused:

PS: I've heard that the MythBusters are no more, unfortunately.

[NSEU]

I think the point is that it will be safe by the time the evac command is given, the doors are opened and the slides fully inflated. No one is going to be able to give you a precise time because there are too many variables.

If your main concern is people getting sucked or blown off the slide during evac, then I can probably think of a few variables

Body mass
Position of the slide in relation to the front/back of the engine
Height of the fences on the slide
Velocity of the person going down the slide
The position of the person on the slide (middle or side)
Size of engine
Wind speed...

Engineers sometimes have to manually open the start engine valves, which involves inserting a manual crank into a hole in the engine cowling during the start process. After the start has been completed (engine confirmed at idle), the engineer moves away from the engine at right angles to the engine. As long as he/she doesn't step past a red stripe painted on the side of the nose cowling, he/she is considered safe. On a 777, the red stripe is only 1.5m (5 feet) from the inlet. When standing directly in front of the engine, there is a safety radius of 4.6m (15 feet). During winds, the margin may increase by 20%. Smaller engines, of course, have smaller safety radii. There is a much larger no-go zone behind the engine, but the risk is lower in the sense that you will only get blown about or over, not sucked into the engine.

[BugSmasher1960]

Thanks NSEU.

It was pretty much just my own curiosity - but I was also thinking in the context of a passenger initiated evacuation when (if) FD crew realise what's happening with "less than" the ideal time to wait.

Was looking at a YouTube video of the GE90-115B attached to their B747 "test bed" - they had the other 3 engines shut down and it was holding it's own - then later on the ground blowing 400lb rocks around - guessing that it's one engine you really don't want to be too close to even at ground idle (probably quite a few you don't want to get too close to, but you get my point hopefully).

[Metro man]

Add burns to being blown over.

[Escape Path]

My guess is that if it was to be of actual concern, i.e. a potential hazard for anyone to be injured/killed, we would have known by now via some recommendation from manufacturers. Even though an engine at idle has some thrust, I'd think that a couple of seconds after you cut the fuel it'll be safe to walk either in front or behind the engine.

In any case, for the A320, our company SOP was to turn off beacon when N1<5%. This was removed from our SOPs some months ago with no further instruction on the matter

[lomapaseo]

Even though an engine at idle has some thrust, I'd think that a couple of seconds after you cut the fuel it'll be safe to walk either in front or behind the engine.

Where's our mechanics in this forum who have to do trim and balance checks?

They must have lots of adhoc data :)

[NSEU]

Add burns to being blown over.

EGT may be 350C(?) or so at idle in the hotstream tailpipe, but if we are talking modern high by-pass ratio engines with mixers, then I can't imagine there will be a great risk. Hairdryer temperatures a metre from the tailpipe a few seconds after shutdown?

[Metro man]

Give it a try and let us know how you get on.;)

[avoman]

Ground engineer here. On the CFM at least after shutdown from idle power the intake draught and exhaust blast drop to non significant levels in a few seconds only

[172_driver]

On the 737 we turn off the beacon as soon as the start lever is moved to cut-off. I do however verify the engine has flamed out first as sticky start lever switches are not unheard of.