spacecadet
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High Ground Thrust While Stopped - Why?

Thu Feb 14, 2013 9:44 pm

Flew back from SXM-JFK the other day on B6 788 and there was a strangely long wait to get to the runway (after having taxied to the final turn before the runway), during which I saw two commuter planes pass us to take off, plus a couple landings. All told we sat holding for about 15 minutes, but the odd part was that the thrust setting during this time was high enough that it was actually shaking the plane as it sat there. I've never experienced that before for such a long period and it was strangely unnerving - it seemed like the pilot had increased thrust to start the taxi, then applied the brakes but never reduced thrust back to idle. Seeing these other planes pass us to take off and sitting there in this loud, shaking plane, I actually started wondering if there was a problem of some kind.

Is there some reason why a relatively high thrust setting would be used while stopped on a taxiway for an extended period of time? I had the thought that maybe they were anticipating needing a quick spool-up for a rolling takeoff in between landings, but we didn't do that; it was a "line up and wait" takeoff. It seems to me that doing this for more than a few minutes would use up a measurable amount of extra fuel, so I figured there had to be some good reason for it and I just wondered what that would be.

[Edited 2013-02-14 13:45:13]
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Goldenshield
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RE: High Ground Thrust While Stopped - Why?

Thu Feb 14, 2013 9:53 pm

Sounds like the plane was too heavy to takeoff, so he was burning off some fuel.
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DarkSnowyNight
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RE: High Ground Thrust While Stopped - Why?

Fri Feb 15, 2013 12:55 am

Burning excess fuel.


I'm a bit surprised this would happen on an A320; the fuel intake flow rate is between 160 - 180 GPM. This is very slow, and almost anything that isn't a 757 will fuel faster, including most RJs.

Most of the time, when burning excess fuel off, it's because the fueler overshot. Going over by between 200 - 500lbs is no big deal and happens on almost every mainline commercial flight. A pilot looking at a final load will generally tell the fueler something along the lines of "great, have a good one" at that point.

However, an overshot of 1000lbs or more will be troublesome. At that point, you definitely have to offload, especially at an airport like SXM, with it's child-size runway. This is a lot slower than even the 320's intake flow rate; almost never more than about 50 GPM. As well, fuel can only be offloaded onto a tanker dedicated to that particular operator. It cannot be off-loaded to a tanker shared with other airlines, and sure as hell not back into the hydrant. So, now we have to schedule a tanker to swing by and offload, assuming, of course, one is available.

Generally, as well, airlines will back-charge a fuel provider something like $1000/min for a delay (this is pretty std & we see the same for MX, groundhandling, etc...). So, at the operations level, there's a hell of a lot of pressure to get that plane out, yesterday.

As you can see, except for very extreme cases, it's better just to cook off a few hundred, or even a thousand, pounds before takeoff. In fact, the only time I ever saw a tanker used to de-fuel an aircraft was in a case where a guy overshot a 757 by 30,000lbs. That's not a typo.
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Max Q
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RE: High Ground Thrust While Stopped - Why?

Fri Feb 15, 2013 1:28 am

That's what it sounds like.


Furthermore, unlike older Aircraft modern ones will 'tell on you' if you exceed a limitation such as maximum take off or landing weight.
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zeke
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RE: High Ground Thrust While Stopped - Why?

Fri Feb 15, 2013 4:34 am

What were the conditions like ? sometime we use higher thrust to shed ice from the engine before takeoff.
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ha763
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RE: High Ground Thrust While Stopped - Why?

Fri Feb 15, 2013 4:58 am

Quoting zeke (Reply 4):

What were the conditions like ? sometime we use higher thrust to shed ice from the engine before takeoff.

OP said it was at SXM. No snow or ice there.
 
spacecadet
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RE: High Ground Thrust While Stopped - Why?

Fri Feb 15, 2013 7:07 am

Thanks for the info - that must have been it. The flight was full and it was a regular SXM day, which means 80 degrees and sunny. I did notice the takeoff roll seemed long-ish as well - I was paying attention to the liftoff points of various flights during the week we were there, in part because our car rental place happened to be right at the spot where both the KLM 747 and Air France A340 lifted off (we saw both at that spot on different days), but the B6 flight to San Juan lifted off a lot further back. We lifted off a bit past our car rental, which means actually beyond the point of the A340 and 747. So the fuel burning explanation makes sense.

Also explains why we were sitting there - even as I wrote my post, I was assuming we'd been waiting for other planes and the high thrust was unrelated, but I guess we were probably sitting there intentionally. The pilots never said anything, but I guess they wouldn't. Shortly before takeoff they did say "we expect it to just be another couple of minutes", which I assumed meant "we're waiting for one more landing aircraft" but probably just meant they were getting close to the target weight.
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maddogjt8d
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RE: High Ground Thrust While Stopped - Why?

Fri Feb 15, 2013 9:11 pm

I was once listening to JFK ATC during a snow storm when one of AA's 757 flights to the caribbean checked in ready to taxi on Ground. He said that the aircraft had taken on lots of extra fuel for deicing and taxiing delays that were anticipated, but never materialized, and as a result they would have to sit on an active taxiway running up both engines for over an hour before they were ready to head to the runway. I thought at the time that was absolutely incredulous, but your explanations about the process of de-fueling an aircraft put this in perspective. Thanks as usual to you awesome TechOps folks!
 
YYZatcboy
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RE: High Ground Thrust While Stopped - Why?

Fri Feb 15, 2013 11:45 pm

Could also have been a mtc item that required an engine run prior to takeoff.
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JAGflyer
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RE: High Ground Thrust While Stopped - Why?

Sat Feb 16, 2013 2:09 am

Listening to YYZ tower during the snowstorm we had last week I heard a lot of chatter about run-ups prior to take off. ATC was asking all the aircraft if they needed to do a run up. Why would they need to do a run-up before taking off on a snowy day? I'd think the engines would easily melt off any snow that touches them while they are idling.
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Starlionblue
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RE: High Ground Thrust While Stopped - Why?

Sat Feb 16, 2013 3:04 am

Quoting JAGflyer (Reply 9):
Listening to YYZ tower during the snowstorm we had last week I heard a lot of chatter about run-ups prior to take off. ATC was asking all the aircraft if they needed to do a run up. Why would they need to do a run-up before taking off on a snowy day? I'd think the engines would easily melt off any snow that touches them while they are idling.

Making sure they are up to the proper temperature before taking off? Making sure they don't quite unexpectedly when at high power setting?
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CosmicCruiser
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RE: High Ground Thrust While Stopped - Why?

Sat Feb 16, 2013 3:12 am

Quoting JAGflyer (Reply 9):
Why would they need to do a run-up before taking off on a snowy day?

for us it was 30 sec at 69% N1 before t/o.
 
atct
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RE: High Ground Thrust While Stopped - Why?

Sat Feb 16, 2013 7:39 am

Quoting JAGflyer (Reply 9):
Why would they need to do a run-up before taking off on a snowy day? I'd think the engines would easily melt off any snow that touches them while they are idling.

They do it up here (Anchorage) mainly due to the cold, not snow. Most of the foreign carriers do a 30 second runup in position then take off.
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CosmicCruiser
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RE: High Ground Thrust While Stopped - Why?

Sat Feb 16, 2013 1:55 pm

typo....meant 60% N1
 
steinberger45
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RE: High Ground Thrust While Stopped - Why?

Mon Feb 25, 2013 3:37 am

Ground holds can be for many things, Acft spacing, paperwork flight plan issues. When you take-off it is determined by what is going on at your destination. If there are delays better to hold on the ground rather than in the air. If you are holding and the pilot has the ac packs on high flow. Cabin noise is louder making you think the plane power setting is above idle.
 
spacecadet
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RE: High Ground Thrust While Stopped - Why?

Mon Feb 25, 2013 5:07 am

Quoting steinberger45 (Reply 14):
Cabin noise is louder making you think the plane power setting is above idle.

Power setting was definitely far above idle. This isn't the first time I've been on a plane holding on a taxiway. But the question of why was answered a while back.
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DashTrash
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RE: High Ground Thrust While Stopped - Why?

Mon Feb 25, 2013 4:10 pm

Quoting JAGflyer (Reply 9):
Why would they need to do a run-up before taking off on a snowy day?

Possibly an anti-ice check.

We had to turn it all on, wait for the "cold" CAS messages, then increase N1 to some percentage and wait for the messages to clear before takeoff. The engines didn't have enough bleed air at low power to keep the sensors from reading a cold signal hence the run up before takeoff.
 
Stabilator
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RE: High Ground Thrust While Stopped - Why?

Tue Feb 26, 2013 1:52 am

Reminds me of a funny little story on a solo flight as a private pilot. Had to make a full-stop up near the MN-Canadian border because fuel wasn't being pulled from one of my Cessna's tanks. After trouble shooting with maintenance on the phone, we agreed I should do a 5min ru-nup on the tank that I was having the problem with. As I started my run-up, a Beech 1900 (belonging to one of the window companies up north, iirc) pulled up next to me. The Pilots had the funniest expression on their faces, along the lines of "WTF is this new pilot doing? It doesnt take 5 minutes to do a run up check on a Cessna".  

Safely made it back to Nodak, thankfully.
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