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EAARbrat
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No more fuel saving measures during taxi?

Thu Mar 05, 2020 7:55 pm

Flying out of ATL today and it dawned on me that aircraft were starting both engines at pushback. What happened to single engine taxi and start up at or near the runway?

Is fuel that cheap?
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BravoOne
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Re: No more fuel saving measures during taxi?

Thu Mar 05, 2020 8:07 pm

What type of aircraft?
 
B757Forever
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Re: No more fuel saving measures during taxi?

Thu Mar 05, 2020 8:16 pm

Some aircraft require engine oil temps to be at operating temperature before takeoff power is applied. If it was the first flight of the day for the aircraft and the engines were cold-soaked overnight they may need run time to get up to temp.
The Rolls Royce Dart. Noise = Shaft Horsepower.
 
Pontius
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Re: No more fuel saving measures during taxi?

Thu Mar 05, 2020 8:18 pm

So many variables in play here, and so little information....

Interesting tidbit: The CFM LEAP takes almost 2 minutes of dry motoring to eliminate rotor bow, 1 minute to light and accelerate to idle power, and then another 3 minutes of warmup before ready for takeoff power. A nearly 6 minute lead time makes single-engine taxi out impractical at many airports.
 
AABusDrvr
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Re: No more fuel saving measures during taxi?

Thu Mar 05, 2020 9:02 pm

The contract talks between the pilots and the company not progressing may have something to do with it. If it was a 737, I won't ever taxi out single engine. It requires too much power on one engine to get moving, you risk blast damage to stuff behind you.
 
strfyr51
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Re: No more fuel saving measures during taxi?

Thu Mar 05, 2020 9:14 pm

a single engine taxi was never that good in the first place Since the engines are great for producing cool air for the Air conditioning system? It isn't that big of a deal to run both engines. Other stuff can help reduce overall fuel burn. Stuff as simple as washing the airplanes and keeping the paint in good order. Keeping the flight controls rigged so you're not using Excessive trim. using better fuel planning with routing. flying optimum routings around Weather can save a bunch of fuel We know a lot more than we have in the past. Not a big deal to taxi on a single engine anymore.
 
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fanoftristars
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Re: No more fuel saving measures during taxi?

Thu Mar 05, 2020 9:17 pm

Seems like they always start both A220 engines at pushback. I've heard SOP for the 777 is start 1 & 2 together, not sure if that's changed. Also, if you're departing the T, A or B gates and taking off to the east, its such a short taxi why not be ready to go.
"FLY DELTA JETS"
 
JoseSalazar
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Re: No more fuel saving measures during taxi?

Thu Mar 05, 2020 9:19 pm

An IAE engined airbus that has been sitting a while requires a 5 minute idle warm up after it is started—1-2 minute start time per engine, that is a lot of time required. With a short taxi, taxiing out on one may not be a good idea. Also, the APU burns about 300 lb/hr, and a NB engine is 600-800 lb/hr at idle. So it’s probably about 5-7 lbs a minute difference, or less than a gallon a minute. At $1.50/gal, that’s not very much difference. Throw in higher thrust required to taxi, more workload/heads down time starting an engine while taxiing and running checklists, it’s not worth it a lot of the time. 45 minute taxi out with a line of 20-30? Yeah probably worth it.
 
alasizon
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Re: No more fuel saving measures during taxi?

Thu Mar 05, 2020 9:20 pm

Also depends on the length of expected taxi, if a crew knows it is a slow time and they can see the threshold from their gate/taxilane, they will often choose to start up both. Single-engine taxi tends to save more fuel going into the hubs than leaving since most aircraft arrive early and the gate is usually occupied.
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EAARbrat
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Re: No more fuel saving measures during taxi?

Thu Mar 05, 2020 9:35 pm

BravoOne wrote:
What type of aircraft?


Spirit A320.

My flight was Frontier A320 Neo.
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Pontius
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Re: No more fuel saving measures during taxi?

Thu Mar 05, 2020 9:54 pm

EAARbrat wrote:
BravoOne wrote:
What type of aircraft?


Spirit A320.

My flight was Frontier A320 Neo.


Then the aforementioned LEAP warmup times apply. Also F9 isn’t usually operating in the middle of a big Delta bank, so their taxi times in ATL are actually pretty short.
 
Woodreau
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Re: No more fuel saving measures during taxi?

Thu Mar 05, 2020 9:57 pm

A 320 NEO (P&W) requires takes 5-7 minutes from the time the engine master is switched from off to on to get to ground idle depending on the required cooling time and requires a 5 minute warm up before takeoff power can be applied.

Because of this, I don’t ever single engine taxi a 320 NEO otherwise I’d hold up the line at the end of the runway waiting for my engine to start before I can even start the clock for engine warmup.

After landing the engine requires a 5 minute cool down before shutting down, so I have to taxi so that I don’t arrive at the gate before the cool down timer ends. Often I arrive at the gate right at the 5 minute mark so I just end up shutting both engines down.
Last edited by Woodreau on Thu Mar 05, 2020 10:01 pm, edited 1 time in total.
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EAARbrat
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Re: No more fuel saving measures during taxi?

Thu Mar 05, 2020 9:58 pm

Starting in the early 2000's I can remember the practice being done on almost all my flights across multiple airlines Domestic and International - A320, CRJ's 737, 757, 767, Mad Dogs for sure. Can't distinctively remember of so on my A330, A340, 747 & 777 flights.
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EAARbrat
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Re: No more fuel saving measures during taxi?

Thu Mar 05, 2020 10:02 pm

Also my home airports in those days were JFK and LGA. Many of my flights were during peak times which could include loooong taxiway waits.

So it makes sense in ATL with all the runways and much shorter taxi times.
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Classa64
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Re: No more fuel saving measures during taxi?

Fri Mar 06, 2020 2:42 am

Pontius wrote:
So many variables in play here, and so little information....

Interesting tidbit: The CFM LEAP takes almost 2 minutes of dry motoring to eliminate rotor bow,


What does this mean?

C.
"Freedom is the miles i'm rolling on"
 
LabQuest
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Re: No more fuel saving measures during taxi?

Fri Mar 06, 2020 2:47 am

This reminded me of something I don't think I've ever seen a 747 taxi out on 2 engines. Is there a reason for this on that specific plane?

Do any 3-4 engine planes taxi with engines off?
 
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Starlionblue
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Re: No more fuel saving measures during taxi?

Fri Mar 06, 2020 4:41 am

Besides warmup times, an issue with reduced engine taxi out is that you have to delay some flows and checklists because you're not done starting up the aircraft until all engines are started. This introduces an additional threat to the operation.

Reduced engine taxi in, on the other hand, does not have this threat because you're going to shut both engines down eventually anyway. Which is why the practice of shutting down one on arrival is far more common than taxiing with one engine on departure.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
Pontius
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Re: No more fuel saving measures during taxi?

Fri Mar 06, 2020 4:52 am

Classa64 wrote:
Pontius wrote:
So many variables in play here, and so little information....

Interesting tidbit: The CFM LEAP takes almost 2 minutes of dry motoring to eliminate rotor bow,


What does this mean?

C.


Shut off the (hot) engine and all the assemblies stop moving. The bottom parts cool quicker, the top stays hot. Thermal differential bends things. Start the engine in the near term and it looks like this:

Image

That's not good. Use the starter to dry (no fuel) motor (turn) the engine, thereby blowing cooling air through it until temperatures normalize across both entire spools.. In a LEAP powered aircraft a pilot can actually watch the engine vibration indication slowly decrease during this time period. Once the FADEC is satisfied, it adds fuel and the engine lights off. Not a new phenomena, it was standard procedure for the first officer on TPE331 powered aircraft to give the propellors a spin after shutdown to avoid the same problem. This is doubtless an imperfect, pilot's eye perspective. Ask lightsaber for the materials.science.
 
spacecadet
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Re: No more fuel saving measures during taxi?

Fri Mar 06, 2020 6:56 am

It depends mostly on the taxi length. This takes into account warmup times, flows, etc. If the pilots know it's going to be a 30 minute taxi, they're going to do it on one engine. If it's 5, they'll do it on both. That's true of pretty much every airliner.

Taxi times aren't usually a surprise. The pilots generally know the taxi time before leaving the gate, both due to the route from the gate to the runway in use and that route length, the amount of traffic ahead of them, whether any deicing or anything is going on, etc. If it's a short taxi and there's not much traffic, they'll start both engines up at pushback, regardless of what airplane it is.
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Aircellist
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Re: No more fuel saving measures during taxi?

Fri Mar 06, 2020 10:54 am

Pontius, I very much like your rotor bow! :thumbsup:
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Tristarsteve
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Re: No more fuel saving measures during taxi?

Fri Mar 06, 2020 12:57 pm

LabQuest wrote:
This reminded me of something I don't think I've ever seen a 747 taxi out on 2 engines. Is there a reason for this on that specific plane?

Do any 3-4 engine planes taxi with engines off?

The Tristar regularly taxied on 1 and 3. Nbr 2 started near the runway.
There was no rotor bow then either!
 
gloom
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Re: No more fuel saving measures during taxi?

Fri Mar 06, 2020 2:00 pm

I guess one engine taxi is mostly used on one occasion - when excess of thrust on idle causes the plane to accelerate.

If the plane does not accelerate on idle, the difference between one engine taxi and two engine taxi is not that much.

Other than that, all engines and planes do have minimum time from start to takeoff thrust, usually somewhere near 5 minutes, as mentioned before. So, unless you taxi 15+ minutes, it's not going to make much of a difference.

To tell the truth, I don't remember one engine taxiing during my flights in Europe, except for props like 8Q400 or ATRs.

Cheers,
Adam
 
cpd
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Re: No more fuel saving measures during taxi?

Fri Mar 06, 2020 7:36 pm

Pontius wrote:
Classa64 wrote:
Pontius wrote:
So many variables in play here, and so little information....

Interesting tidbit: The CFM LEAP takes almost 2 minutes of dry motoring to eliminate rotor bow,


What does this mean?

C.


Shut off the (hot) engine and all the assemblies stop moving. The bottom parts cool quicker, the top stays hot. Thermal differential bends things. Start the engine in the near term and it looks like this:

Image

That's not good. Use the starter to dry (no fuel) motor (turn) the engine, thereby blowing cooling air through it until temperatures normalize across both entire spools.. In a LEAP powered aircraft a pilot can actually watch the engine vibration indication slowly decrease during this time period. Once the FADEC is satisfied, it adds fuel and the engine lights off. Not a new phenomena, it was standard procedure for the first officer on TPE331 powered aircraft to give the propellors a spin after shutdown to avoid the same problem. This is doubtless an imperfect, pilot's eye perspective. Ask lightsaber for the materials.science.


One aircraft had a setting for engine start to run the engines at a low speed to allow the temperatures to even out (debow).

That was back in the old days.
 
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Starlionblue
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Re: No more fuel saving measures during taxi?

Fri Mar 06, 2020 11:39 pm

gloom wrote:
I guess one engine taxi is mostly used on one occasion - when excess of thrust on idle causes the plane to accelerate.

If the plane does not accelerate on idle, the difference between one engine taxi and two engine taxi is not that much.

Other than that, all engines and planes do have minimum time from start to takeoff thrust, usually somewhere near 5 minutes, as mentioned before. So, unless you taxi 15+ minutes, it's not going to make much of a difference.

To tell the truth, I don't remember one engine taxiing during my flights in Europe, except for props like 8Q400 or ATRs.

Cheers,
Adam


Even if you have to add a bit of thrust when taxiing on one engine, it will be cheaper than two. It isn't only fuel, but also maintenance cost since many airlines pay by the hour. Certainly at high weights you might not want to taxi on one engines, but in most other cases there is enough thrust at idle except for starting to move and some turns.

According to repeated missives from our management, even one minute of single engine taxi results in savings. We do single engine taxi on Europe flights all the time. The only times we don't is if the taxi in is very short (engine cooling), if the airport regulations forbid it, or if company guidelines forbid it (e.g. an airport with very rough and uneven taxiways).
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
strfyr51
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Re: No more fuel saving measures during taxi?

Mon Mar 09, 2020 1:02 am

[quote="LabQuest"]This reminded me of something I don't think I've ever seen a 747 taxi out on 2 engines. Is there a reason for this on that specific plane?

Do any 3-4 engine planes taxi with engines off?[/quo
Pushing a fully loaded 747 out on 2 engines is possible but getting Breakaway thrust on just 2 engines? Might damage an airplane in Trail behind them before they get moving suitably. It might not be a good idea.

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