A Question for the Pilots Here.

[Magiver]

@Habeed

So narrowing down to the original question, for “action movie” heroics, it sounds like the main limitation for starting the bigger jets like a 767 is the spool up times for the engines.

No they still have to go through a lengthy checklist which involves more than just flipping switches. They have to review all aspects of the flight for weather and match up their fuel loads and trim settings.

@Habeed

I don’t suppose there’s a way to short-cut that, such as starting both engines at the same time? Is that even possible or does the electrical system not have enough capacity to perform such an action?

on a 4 engine plane they can and do start more than one engine at a time. Not sure if you can speed that up much with an airstart because then the airstart has to be disconnected and moved out of the way. Without external air they have to start 1 engine first to get enough bleed air to start the others. Each engine then has to slowly spin up to the correct operating temperature. You can’t really speed that part of it up without creating a hot start which is unburnt fuel igniting. Very impressive and not particularly good for the engine. So it would be something like engine 1, 5 minutes. Engine 2,3 five minutes and within that 5 minutes engine 4.

@Habeed

I remember reading the B-52s have a solid rocket motor “start cartridge” that injects hot gas into the engine to get them to start up in under a minute, and they could do this to all 8 engines simultaneously.

There’s a horde of zombies/bad guys after you, so you need to get the aircraft moving pronto. Assume the 767 is in good condition and if you don’t get it going the baddies will kill everyone on the plane anyway.

Gone in 10. And this isn’t a hypothetical in bad parts of the world. Any seasoned freight dog will tell you stories of riots at the perimeter of an airport. Fuck the weather briefing and any part of the checklist that doesn’t involve getting the plane off the ground. We’re leaving. NOW.

[Habeed]

@Magiver

No they still have to go through a lengthy checklist which involves more than just flipping switches. They have to review all aspects of the flight for weather and match up their fuel loads and trim settings.

Again, it’s an emergency. You must escape the airport. You just assume the fuel load is correct and the trim is close enough to allow for flight.

[LSLGuy]

Some misinformation on big-jet realities just above.

Older 747s can start 2 at a time. I don’t know for sure about the latest models. And big turbofans all take about 45 seconds apiece. So you could have all 4 running in 90 seconds from beginning the first start.

On a 737, 757, or 767 the system is limited to starting one at a time. But it’s mechanically possible to overlap the two starts more than we normally do.

Schematically speaking, all jet engines start the same way. First a starter motor spins the engine up to a speed well below idle. Nowadays almost all starters are pneumatic, but some are electric. Then, while leaving the starter motor engaged, add fuel & ignition. A couple seconds later the fire starts and begins to accelerate the engine, assisted by the still-running starter. As the engine gets to around 80% of idle speed, cut off the starter motor & allow the combustion bootstrapping to finish the acceleration to stabilized idle.

Then we check all the gauges to ensure everything has settled down properly, followed by reconfiguring for the next start or for post-start taxi ops as appropriate. Once the engine is idling, we *could *immediately go to takeoff power; normally we let it warm up at idle or near idle for a few minutes; the typical taxi process is more than sufficient time.

In 1950s & 1960s airplanes pilots and flight engineers manually did all the sequencing of valves & switches & did all the checking. In brand new airplanes you flip one switch from engine-off to engine-on and the computer does everything else, including handling any malfunctions.

The stuff I fly is in the middle. Sequencing is auto, but all monitoring is manual. Modern engines start very reliably. But if they do fail to start properly we can spend a couple million dollars in just a few seconds through inattention to the developing problem. So it’s done slowly and carefully with two sets of undivided attentions and much measure twice cut once procedures.

[Magiver]

@Habeed

Again, it’s an emergency. You must escape the airport. You just assume the fuel load is correct and the trim is close enough to allow for flight.

Commercial plane, gone in 10 minutes. In the air. If the angle of attack is not critical then flap setting just needs to be close enough to get off the ground.

In my little plane I’m in the air in 10 seconds. I’d have it started before the door was shut and take off from the ramp or taxiway.

[Magiver]

@LSLGuy

Some misinformation on big-jet realities just above.

Older 747s can start 2 at a time. I don’t know for sure about the latest models. And big turbofans all take about 45 seconds apiece. So you could have all 4 running in 90 seconds from beginning the first start.

I’ve watched 747-200, 400, and 800’s start under varying degrees of desire to leave quickly. I’ve never seen all 4 running in 90 seconds and it normally takes minutes to properly spool them up. You could taxi off of #1 and with the normal time it takes to get to the runway have them all running.

Never asked a pilot how fast they could go to full throttle if they didn’t care about long term use of the engines.

[LSLGuy]

@Magiver

I’ve watched 747-200, 400, and 800’s start under varying degrees of desire to leave quickly. I’ve never seen all 4 running in 90 seconds and it normally takes minutes to properly spool them up. You could taxi off of #1 and with the normal time it takes to get to the runway have them all running.

Never asked a pilot how fast they could go to full throttle if they didn’t care about long term use of the engines.

I think we’re talking past each other a bit. As you rightly say, a couple minutes apiece is about par for routine ops, even if we’re in a typical workaday hustle.

Starting 2 at a time was originally intended as a routine but expedited procedure on the 747, although darn few operators actually ever did it. It required the two APU installation, which also not that many operators ever bought.

90ish seconds total and no warm up time before full power are all zombie apocalypse escape moves. Not something you’d ever see in the real world. There are safer places in the whole preflight-through-takeoff process to shave time when needed.

Spool-up to idle time is what it is; that can’t be rushed. What can be hurried along is overlapping the starts and adding fuel at the bare minimum RPM values vice the higher values we normally use to provide cooler starts & longer engine life.

[Morgenstern]

To actually take off, without care or caution to a checklist safety or even sanity, (VERY unwise, and probably never done) these steps. Untie and remove chalks.

1. Master on.
2. Fuel on both (or fullest tank)
3. Mixture rich.
4. Engine start.
5. Release brake
6. Throttle in.
   Take off.

Never mind the engine is still too cold, but in this scenario the mafia is chasing you with machine guns blazing, so you have no choice.

[Ike_Witt]

Reading these checklists is a pretty good indication of why we will never have flying cars.

[Leo_Bloom]

I sound like Emily Litella here, but what’s all this about “mixture” and “mixture rich?” What’s that? I thought if the needle’s at F, you’re good to go.

[Richard_Pearse]

On a piston engined aeroplane you need to manually adjust the fuel/air ratio going to the engine. This is normally done with a red knobbed lever in the cockpit. Full rich gives max fuel and is used for max power at sea level, so for take-off and landing (full power is needed in case you go around) you need to check the mixture is rich. Once in the cruise you lean the mixture in order to get better fuel economy.

It doesn’t apply to all aircraft, a Tiger Moth has no mixture control and I guess other aircraft of that era wouldn’t either.

[Johnny_L.A]

@Ike_Witt

Reading these checklists is a pretty good indication of why we will never have flying cars.

It’s also a pretty good indication of why there are checklists.

@Leo_Bloom

I sound like Emily Litella here, but what’s all this about “mixture” and “mixture rich?” What’s that? I thought if the needle’s at F, you’re good to go.

‘Mixture’ is the ratio of air to fuel. To achieve full power, the mixture must be leaned or enriched depending on density and altitude. For example, air is less dense at high altitudes, so the ratio is lower than at sea level and the amount of fuel being added to the mixture must be lessened. i.e., you ‘lean’ the mixture. ‘Full rich’ means that you are putting the maximum amount of fuel into the mixture.

Air-fuel ratio

[Leo_Bloom]

Thanks to both. Had no idea.
Also, I would bet using “lean” as a transitive verb without a preposition is unique to you guys. Never heard that before as well.

[Habeed]

@Richard_Pearse

On a piston engined aeroplane you need to manually adjust the fuel/air ratio going to the engine.

This is only because a mixture of high engineering costs, high government regulation, and high liability makes developing an engine with digital mixture controls extremely expensive, right? (although I think newer model single engine general aviation planes have them)

Basically, this task you have to do with old piston engined aircraft is the same job that a flight engineer had to perform with earlier jet aircraft.

Just to show how technology shifts, that task that LSL described of carefully manually monitoring an engine? It is technically possible to automate that task and you could use some kind of expert system (maybe using neural nets) to predict engine problems based on past historical data from a particular model engine/aircraft conditions. You could even have your aircraft upload it’s engine telemetry data to a central server whenever it is at an airport and use this historical data to improve your predictions. Ultimately, a software system like this could easily beat humans in predictive capability for engine problems. (just like how today, google can find information faster and better than any human being who has ever lived)

[LSLGuy]

@Habeed

This is only because a mixture of high engineering costs, high government regulation, and high liability makes developing an engine with digital mixture controls extremely expensive, right?

…

Just to show how technology shifts, that task that LSL described of carefully manually monitoring an engine? It is technically possible to automate that task and you could use some kind of expert system (maybe using neural nets) to predict engine problems based on past historical data from a particular model engine/aircraft conditions. You could even have your aircraft upload it’s engine telemetry data to a central server whenever it is at an airport and use this historical data to improve your predictions. Ultimately, a software system like this could easily beat humans in predictive capability for engine problems. …

Quite right.

For most lightplane-related stuff, the combination of a numerically tiny market and FAA-enforced technological stasis at 1950s level technology largely prevents progress.

Conversely, at the other end of the spectrum …

We have exactly that sort of engine monitoring on all our newer big jet fleets. And it all gets datalinked or sneaker-netted off the airplane periodically. Predictive maintenance is one of the big watchwords for cost savings in the next 5-10 years. Engines are leading the way, but the whole rest of the systems on the newest aircraft are also being hooked up the same way.

But it’s definitely a matter of spending $100K per aircraft to save $1Mil per aircraft. It doesn’t scale down to lightplane operators each spending $100 to save $1000.

[Johnny_L.A]

The Porsche PFM (Porsche F[/b[lugm**otoren) had single-lever operation that combined throttle, mixture, and propeller pitch. Porsche surrendered the type certificate in 2007, and would no longer support the engine. The FAA said that airplanes with the PFM 3200 would remain valid as long as the engines met FAA maintenance and operation requirements. With no replacement parts forthcoming, I’m wondering how those Mooney drivers will continue flying when their engines wear out. Get FAA approval to use non-certified Porsche parts? Aftermarket parts? Would this require re-classifying the airplanes into the Experimental category? Or can they retrofit Continental TSIO-550s with an STC?

[Habeed]

@Johnny_L.A

The Porsche PFM (Porsche F[/b[lugm**otoren) had single-lever operation that combined throttle, mixture, and propeller pitch.

Mixture settings (how much air to fuel) is a function of altitude, air temperature, aircraft speed, and a bunch of other variables, as well as throttle position.

Propeller pitch is a function of aircraft speed, air pressure, and engine power levels, which are governed by the throttle lever.

So, I take it the way the Porsche plane works is that the pilot sets the throttle, which determines the desired power output, and then the aircraft computer sets everything else?

Another issue here is a UI problem. Should “50%” on the throttle lever mean :

50% of available power under these conditions OR
50% of RATED power per the engine’s maximum power output under normal conditions?

Essentially, if the engine can produce 100 horsepower but right now can only produce 80 HP because of lower air pressure or it’s too hot, etc, then should 50% throttle produce :

50 HP or 40 HP.

If you do it the former way, that means that pilots can probably learn their aircraft better, since the aircraft would perform the same at a given throttle setting, assuming it is not set so high for current conditions. However, that means there’s going to be a “dead zone”, where say between 80% on the throttle lever and 100% you won’t get any more power. Also, you need an extra button to indicate “maximum possible power” because if conditions are such that the 100 HP engine can develop 120 HP at the current air temperature/pressure, then the pilot needs a control to tell the plane to “give me everything”.

If you do it the latter way, that’s probably how most current aircraft perform. The throttle lever controls the air intake vanes so 100% throttle means “wide open”.

LSL, how do the big jets do it? Everything I said above applies to big jets, although they lack propeller pitch those engines must have dozens of internal controls that can be adjusted.

[Johnny_L.A]

Basically, by the time you get up to 5,000 feet you’re only able to make 75% of rated power at full throttle for a normally-aspirated engine.

[Morgenstern]

@Leo_Bloom

Thanks to both. Had no idea.
Also, I would bet using “lean” as a transitive verb without a preposition is unique to you guys. Never heard that before as well.

One more thing. With a car, you turn off the ignition to stop the engine. With an aircraft, (at least a Cessna 172 and most light aircraft) you pull the mixture back to idle cut off. This starves the engine of fuel and causes it to stop running. You switch off the ignition until after the engine has stopped running.

[Magiver]

Here’s where technology meets reality. Do you want a FADEC system (Full Authority Digital Engine Control) on your plane or magnetos and a couple of levers that don’t know nuttin about failure.

Yeah granted you can keep a magneto around as a backup but that still leaves the fuel injection system. For everyday flying it’s best to have the simplest system available. Throw a little oil on the cables once a year and it’s happy.

FADEC systems make more sense on twin engine planes and turbocharged planes.

[Senegoid]

And at the far other end of the technology spectrum,
speaking of Schweizer mochines . . .

Okay, here’s what it takes to get a Schweizer 2-33 in the air, starting from full power-off as OP specifies.

[ul]
[li] Preflight the aircraft: The usual stuff: Fiddle with ailerons, elevator, rudder, look for loose nuts and bolts, visually check tire and skid for excess wear, etc.[/li][li] At some point, this involves opening the cockpit. There’s a lever you have to move to do that.[/li][li] Know your weight. If have passenger, ask passenger’s weight. [/li][li] Estimate ballast required for weight. Stick appropriate sized lead brick in cockpit.[/li][li] Pull aircraft onto runway.[/li][li] Hop in. Passenger too, if present. (Buckle up. Although OP specified not to count that.)[/li][li] Line crew hooks up rope. [/li][li] Line crew gives rope good sharp tug to make sure it’s secure.[/li][li] Tare the altimeter. :eek: (Optional; common dirty trick for strictly local flights.)[/li][li] Make sure spoilers closed. Shove lever if needed.[/li][li] Meanwhile, towplane lines up on runway. Line crew hooks up rope.[/li][li] Point out doggie bag to passenger.[/li][li] Ready? Set? Everybody happy? Then wag your tail. (This must have something to do with having a doggie bag on board.)[/li][*] Close cockpit. Shove some lever to lock it. (Note, typically close cockpit after starting to roll! :eek:)[/ul]