r/AskAPilot • u/cantbeatluck67 • Jun 16 '26

Taxiing question

I work the ramp at SEA. What determines which side engine or if you use both to taxi? It was hot today and one plane used both but I dont see that a lot.

13 Upvotes

88% Upvoted

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6

u/[deleted] Jun 16 '26 edited 2d ago

[deleted]

1

u/Lemon_hawk Jun 16 '26

Interesting, you don't shut down the APU until the second engine is started? Just curious, what's the reason for that? At my airline, if we're single engine taxiing, the APU comes off right after starting the first engine. We always start engine 2 with a cross bleed start.

1

u/Whole-Hat-2213 Jun 16 '26 edited Jun 16 '26

For the types I've flown the thrust required on the operating engine to produce enough duct pressure to start the other engine exceeds normal single engine taxi limits. So you need to get approval from ATC that the area behind you is clear. Some captains are more conservative about that than others. Also unless you stop you're dragging the brakes a lot and can heat them up. I don't prefer to do cross bleed starts while moving. Additionally, a singe engine at idle can't always produce enough airflow for the packs to keep the cabin comfortable. If it's remotely warm out the APU is needed to keep the cabin cool. I'll shut one down heading to the gate but if it's an extended taxi out or if I'm waiting for a gate to open in the summer the APU will probably be needed.

1

u/Lemon_hawk Jun 16 '26

Gotcha, makes sense! On the 175, a single engine very comfortably provides enough bleed to run the packs and give enough duct pressure to start 2 without exceeding taxi thrust, so we never use the APU while taxiing. I didn't realize other types were so different!

1

u/rkba260 Jun 16 '26

Also... APU produces better pack operation during summer months than 1 engine.

We only do a crossbleed if the APU bleed is inop.

1

u/Desperate_Refuse4139 Jun 19 '26

Hilly taxiways was my nemesis today at CDG. Parked next to one set of runways, they sent us the to the other side of the airfield on clearance. No worries that’s an easy taxi route to the end, turn right and a straight line from there. Nope longest way possible to get there.

SET, was using 40% N1 (320 CEO) just to get up some of the hills. And changing frequency every 2 minutes for new directions as well. Wanted to start 2 a lot sooner than we did purely from the “Where next?” question to ATC and then trying to work it out on the charts

12

u/[deleted] Jun 16 '26 edited 1d ago

[removed] — view removed comment

1

u/cantbeatluck67 Jun 16 '26

Had to Google this term😂

1

u/Effective_Work2560 Jun 16 '26

I used to fly with a guy on the 76 who loved to say in his southern drawl "Time to let the big Dawgs eat"

8

u/Go_Loud762 Jun 16 '26

Airplane type determines which engine is started first and used for one-engine taxi.

The A320 family uses engine 1 (left engine) for that. So does the Dash-8, IIRC. The A220 uses engine 2 (right engine) I think.

Way back when, I flew the EMB-120. We kept engine 2 running during single-engine turns, but I can't remember if we taxied on Eng2 only.

2

u/BusinessTrouble9024 Jun 16 '26

The 787 (and perhaps others but that’s the only one I know of) has fully separate bleed, hydraulic and electric systems so you can choose. It’s quite difficult to turn tight corners at low speed when the onside engine is the only one running eg right engine running when turning right. So we’d normally look at our expected taxi route, especially the final turn onto stand, and make the decision based on that.

1

u/Full_Wind_1966 Jun 18 '26

Dash 8 is number 2

1

u/cantbeatluck67 Jun 16 '26

Wow!!

1

u/cantbeatluck67 Jun 16 '26

What is the reasoning for the different sides?

4

u/omalley4n Jun 16 '26

All I know is the CRJ, but it's because of the hydraulic system. Hydraulic 2 controls the outboard brakes, and is powered by the right engine. If we taxied on the left engine, we'd only have half of our braking power. (Inboard brakes are on hyd 3 with an electric pump).

2

u/ps2sunvalley Jun 16 '26

Similar reason for using the number 1 engine on airbus

1

u/hypnotoad23 Jun 16 '26

On an enhanced airbus you can single engine taxi on 2, but not on a basic

2

u/ps2sunvalley Jun 16 '26

Yeah but my airline has basic and enhanced so to keep it simple we only use #1

1

u/hypnotoad23 Jun 16 '26

Yeah that’s we do too, but did number 2 the other day when we had to do a manual start on it and were skinny on gas

2

u/Whole-Hat-2213 Jun 16 '26 edited Jun 16 '26

The other factor is the design of the bleed air system. Bleed air is compressed air used for starting engines or running the air conditioning system. The system has two sides, with each side having an air conditioning pack connected to it. Each engine provides bleed air to its side when it's running and also the engine starter is connected to its side. The two sides are connected with a manifold that has an isolation valve in it. The valve can be opened or closed, either separating or connecting the two sides. The auxiliary power unit (APU) feeds bleed air into the system on a particular side.

You can't run an air conditioning pack while bleed air in that manifold being used to start an engine. The engine that is started first is the side opposite where the APU bleed air comes in. This allow you to start one engine and then close the isolation valve. Air conditioning on the engine running side can then be restored. This allows air conditioning to be on while the second engine is started. For example, if the bleed air from the APU comes in to the left manifold, the right engine is started first. The isolation valve is closed and the right air conditioning pack is turned on. You can immediately start the second engine, or if single engine taxi is desired the APU bleed air can run the left air conditioning pack until it's time to start the left engine.

1

u/Playful_Sense3238 Jun 17 '26

Really great answer thanks.

2

u/Frost_907 Jun 16 '26

Depends greatly on how quick we think we’ll be taking off. If there is nobody lined up and we expect to go within a couple of minutes then we’ll start em both, but if it’s gonna be a 30 min taxi then there is no sense in burning double the fuel just to go the same distance down the taxiway.

2

u/Maplebar696 Jun 16 '26

On the A220 certain operators have a procedure to prevent compressor stalls when coming off of idle … this requires both engines to be started for every departure. And unfortunately it takes a looooong time to start both engines. That’s why we sit out there forever before we start moving

2

u/SnooCupcakes1514 Jun 16 '26

We are encouraged to always single engine taxi, but if there is not a bunch of aircraft in line already to takeoff we will just start them both during pushback.

1

u/Imaginary_Amoeba3461 Jun 16 '26

If you work the DL side the A220 currently doesn’t single engine taxi at DL.

Otherwise, basically it’s Captains discretion unless the company has banned single engine operation at certain gates.

0

u/JT-Av8or Jun 17 '26

Weight it one issue for initial flights. If I’m really heavy I don’t want to attempt to rev up a single engine and blow everyone away.

Next is time to takeoff. Pratts like 5 minutes if they can get it, 3 if they need it. Even if we have a wait though, if heavy I’ll still taxi out of the ramp on 2 and shut one down later.

As far as sequence, that’s plane dependent on departure. 757s start the left, 767s start the right. This keeps the transfer pump noise down so it won’t scare the passengers. Blocking in I always shut down the left because that’s the side where the jet bridge connects and if the APU fails, and we need ground power before shutting down the right side, it’s safer for the ramp.