11 minutes ago, Chock said:

If you are including older 737s in the mix, back before noise abatement etc, it was not unknown for jets to back themselves out of the gate using their clamshell thrust reversers, although it was more common with T-tails with their engines at the back. This is known as a powerback. Stuff like DC-9s and 727s and occasionally 737s would do that and it was approved by the manufacturers on some types, but I suspect it wouldn't be tolerated these days, not least because of the risk of foreign object damage possibilities with high bypass turbofans, and damage to the terminal too lol. But of course when that did used to happen with low bypass turbojets, it would mean the engines had been cranked up at the gate. You can still see some turboprops do powerbacks from time to time.

Really? Sounds like a really dangerous procedure... for the reasons you've already mentioned.

22 minutes ago, Chock said:

If you are including older 737s in the mix, back before noise abatement etc, it was not unknown for jets to back themselves out of the gate using their clamshell thrust reversers, although it was more common with T-tails with their engines at the back. This is known as a powerback. Stuff like DC-9s and 727s and occasionally 737s would do that and it was approved by the manufacturers on some types, but I suspect it wouldn't be tolerated these days, not least because of the risk of foreign object damage possibilities with high bypass turbofans, and damage to the terminal too lol. But of course when that did used to happen with low bypass turbojets, it would mean the engines had been cranked up at the gate. You can still see some turboprops do powerbacks from time to time.

I've marshalled on powerbacks on DC-9's, 727's and even a Saab340 but the worst was a 737-200. He was blocked in by an inop GPU and we didn't use pushbacks, so in the interest of try to get the flight out in a timely manner, I asked the captain if he wanted to try a powerback. He said sure. Well, once he opened the thrust reversers and added power, the aircraft backed up but it also peeled up asphalt as it went. It made a huge mess which we spent the rest of the afternoon cleaning up. Fortunately, management never found out what I had done.

5 hours ago, J van E said:

 

 

Which leaves the question: when to start the engines. Engine 1 on GPU and engine 2 on APU using pushback? Or engine 2 and then engine 1 on APU during pushback? I don't suppose anyone would start BOTH engines on GPU, let alone APU?

The purpose of the GPU is to provide electric power to the aircraft, and probably 95 percent of the time, aircraft at the gate will be connected to ground power. As others have pointed out, many airports have restrictions on how long an APU can be operated on the ground due to concerns about noise and air pollution.

A GPU can take the form of a stand-alone power cart - typically powered by a Diesel engine - but larger airports usually have power cables right on the jet bridge. Somewhere in or near the jet bridge will be a power converter to change the local mains AC power to the 3 phase, 115 volt, 400 Hz power that aircraft require.

However, jet aircraft cannot be started using just an electric GPU. The engine starters are driven by high pressure bleed air, and the normal source of the required compressed air is the aircraft’s own APU, so the typical scenario will be for the APU to be started a few minutes before pushback. Once the APU generator is online, the gate GPU power will be disconnected, and during pushback, the APU bleed air will be used to start the engines.

Larger airports do have ground carts capable of supplying high pressure air for those aircraft whose APU may be inoperative, but their use is not typical. An air cart (commonly called a “huffer”), may use a large air tank to supply the required airflow, or may be essentially an “APU on wheels”, with a APU-style jet engine on board capable of supplying the required bleed air.

The Concorde is the only commercial aircraft I can think of that was ALWAYS started with a ground air cart, because it had no APU.

The only airliners in common use that can be started electrically are turboprops. Smaller biz jets often use electric DC starter generators, but transport-category jets, from the CRJ or EMB 135/145 on up always use air-driven starters.

The large-diameter air hoses that are often connected to airliners parked at the gate are used to supply conditioned air for cabin heating or cooling. That air supply cannot be used for starting engines.

28 minutes ago, JRBarrett said:

The purpose of the GPU is to provide electric power to the aircraft, and probably 95 percent of the time, aircraft at the gate will be connected to ground power. As others have pointed out, many airports have restrictions on how long an APU can be operated on the ground due to concerns about noise and air pollution.

A GPU can take the form of a stand-alone power cart - typically powered by a Diesel engine - but larger airports usually have power cables right on the jet bridge. Somewhere in or near the jet bridge will be a power converter to change the local mains AC power to the 3 phase, 115 volt, 400 Hz power that aircraft require.

However, jet aircraft cannot be started using just an electric GPU. The engine starters are driven by high pressure bleed air, and the normal source of the required compressed air is the aircraft’s own APU, so the typical scenario will be for the APU to be started a few minutes before pushback. Once the APU generator is online, the gate GPU power will be disconnected, and during pushback, the APU bleed air will be used to start the engines.

Larger airports do have ground carts capable of supplying high pressure air for those aircraft whose APU may be inoperative, but their use is not typical. An air cart (commonly called a “huffer”), may use a large air tank to supply the required airflow, or may be essentially an “APU on wheels”, with a APU-style jet engine on board capable of supplying the required bleed air.

The Concorde is the only commercial aircraft I can think of that was ALWAYS started with a ground air cart, because it had no APU.

The only airliners in common use that can be started electrically are turboprops. Smaller biz jets often use electric DC starter generators, but transport-category jets, from the CRJ or EMB 135/145 on up always use air-driven starters.

The large-diameter air hoses that are often connected to airliners parked at the gate are used to supply conditioned air for cabin heating or cooling. That air supply cannot be used for starting engines.

I could be wrong but I think that 707's and maybe even DC-8's didn't have APU's and required an air start.

10 minutes ago, mwilk said:

I could be wrong but I think that 707's and maybe even DC-8's didn't have APU's and required an air start.

I was thinking more of current-model airliners. I do think that an APU was an option for later versions of the DC-8, but the first ones indeed had no APU.

The Concorde, of course, has not flown in almost 14 years, but was the only aircraft that came to mind still flying in the “modern” jet era that always required ground air to start.

I did watch a Natita-bound Delta 747-400 being started using an air cart at Detroit earlier this year. (I assume the APU was out of service). The outboard engines were stated at the gate, then it was pushed back to do a crossbleed start of the inboards once it was on the taxi lane.

2 hours ago, Chock said:

Stuff like DC-9s and 727s and occasionally 737s

Also the F100.

blaustern

7 hours ago, JRBarrett said:

The only airliners in common use that can be started electrically are turboprops. Smaller biz jets often use electric DC starter generators, but transport-category jets, from the CRJ or EMB 135/145 on up always use air-driven starters.

I am currently flying (or learning to fly) the Dash Q400. The tutorials don’t talk about turning the APU bleed on during the preparation and before start: so this is because the plane is started electrically? 

16 hours ago, J van E said:

I am currently flying (or learning to fly) the Dash Q400. The tutorials don’t talk about turning the APU bleed on during the preparation and before start: so this is because the plane is started electrically? 

I’m not sure what the official SOPs are for the Q400. In most of the APU-equipped biz jets I have worked on (that start electrically), the APU bleed is used prior to engine start to run the pack (or packs) for cabin air conditioning, but the bleed is switched off just before engine start.

When an electric engine starter is engaged, the load on the APU generator increases significantly, (easily peaking at 500 amps or more), which causes the APU EGT to rise. Trying to supply more torque to the generator and bleed air to the packs could cause the APU to over temp.

I would think that on the Q400 a similar procedure would be followed. APU bleed used for air conditioning at the gate at the pilots’ discretion, but shut off prior to engine start.

On an aircraft with air-driven starters, the APU bleed supply has to be “on” of course, in which case the packs are manually or automatically shut off at engine start. There is no significant change in the APU electrical load during start on these aircraft as there would be on an aircraft using electrical starters.

23 hours ago, JRBarrett said:

In most of the APU-equipped biz jets I have worked on (that start electrically), the APU bleed is used prior to engine start to run the pack (or packs) for cabin air conditioning, but the bleed is switched off just before engine start.

When an electric engine starter is engaged, the load on the APU generator increases significantly, (easily peaking at 500 amps or more), which causes the APU EGT to rise. Trying to supply more torque to the generator and bleed air to the packs could cause the APU to over temp.

I would think that on the Q400 a similar procedure would be followed. APU bleed used for air conditioning at the gate at the pilots’ discretion, but shut off prior to engine start.

Thanks, interesting information.