Ask The Pilot

[jb747]

I refer to the question of aircraft braking. Here is the scenario:
Your aircraft is an A380 fully loaded and with fuel for 14 hours flying. Your route is SIN to LHR.
You have started your takeoff roll and when just short of V1 the outer starboard engine starts to vibrate violently.
In order of priority what actions would you take? What would the effect be on the aircraft? Would you attempt to get off the runway?

If you chose to abort....

Say STOP. Slam the thrust levers closed. Ensure that spoilers rise and auto brake activates. If it doesn't max manual braking. FO will call what he sees, and tell the tower when you stop. Then ECAM if you have anything, and get the EVACUATION checklist ready to go.

At low speed, just before you stop, turn to place any suss engine downwind. The tyres will deflate. Stay on the runway.

But, unless the engine gave you a swing, you're unlikely to abort at high speed for an unrecognised vibration....better to take it flying.

 

[Quickstatus]

...though we all still seem to end up deaf

Is hearing loss an occupational hazard for pilots and cabin crew?

Apparently there were more than two EK events..

DME/JFK/?

 

[jb747]

I don't know about cabin crew, and they don't have to go through medicals like the pilots anyway. But, 40 plus years of association with aircraft seems to affect most of us.

 

[lastrow]

Some engines have some interesting internal rotation. I've never followed it up, but apparently the Trents that I fly have spools rotating in different directions. The Airbus A400 military transport, has props that rotate in different directions....

The question of rotation is very important as it can affect harmonic vibration. A motor cruiser with dual propellers for example, should have each one rotating in a different direction (contra rotating) . Some stern drives have two concentric contra rotating propellers, as do some propeller driven aircraft , Russian for example.
Getting back to jet engines I expect that careful attention is given is given to harmonic minimisation and this would result in different rotational directions of engine components and of the engines themselves.

 

[Quickstatus]

The question of rotation is very important as it can affect harmonic vibration.
Getting back to jet engines I expect that careful attention is given is given to harmonic minimisation and this would result in different rotational directions of engine components and of the engines themselves.

The 787 Trents are about to have reduced ETOPS limits imposed due to this particular issue - the harmonics of one part of the compressor resonate at a particular high thrust setting combined with certain temperature and altitude conditions resulting in premature failure

These Trent 1000 also have contrarotating compressor spools - apparently improves engine efficiency.

.....

I don't know about cabin crew, and they don't have to go through medicals like the pilots anyway. But, 40 plus years of association with aircraft seems to affect most of us.

Some studies suggest hearing loss is noise induced and left ear more hearing loss than right ear. Frequency most affected 4-6kJz

 

[jb747]

Getting back to jet engines I expect that careful attention is given is given to harmonic minimisation and this would result in different rotational directions of engine components and of the engines themselves.

Whilst I'm sure harmonics are a large part of the job of designing aircraft and engines, the engines do not rotate in different directions. They are totally interchangeable.

Spool rotation is a different matter. Doing a bit of a search, and it appears that multi spool engines mostly rotate in the same direction, with the Trent for the A380 being the first exception. In the military world, the Harrier used different directions to help damp gyro effects.

 

[lastrow]

But, unless the engine gave you a swing, you're unlikely to abort at high speed for an unrecognised vibration....better to take it flying.

If you chose the flying option what would you do about the vibrating engine? If you immediately shut it down can three engines take a fully laden A380 aloft?
Bearing in mind that you have fuel for 14 hours on board simply going around would not be available due to landing weight.
I assume you would have to dump a great amount of fuel whilst flying around on three engines. how long would this take? Would you simply circle with a view of landing back at your originating airport or would you take some other course of action? .

 

[jb747]

If you chose the flying option what would you do about the vibrating engine? If you immediately shut it down can three engines take a fully laden A380 aloft?

Of course they can. If they couldn't there would be no point to having a V1 would there.

Bearing in mind that you have fuel for 14 hours on board simply going around would not be available due to landing weight.

Just because I'm above max landing weight, it does not mean I can't land. Varying levels of maintenance action will be required...but a rule of thumb is that any runway you can take off from...you can immediately land on.

I assume you would have to dump a great amount of fuel whilst flying around on three engines. how long would this take? Would you simply circle with a view of landing back at your originating airport or would you take some other course of action?

I may have to dump a lot of fuel. I don't necessarily have to. The circumstances of the day will control that. If I had a problem out of Melbourne, and the weather there was not conducive to a return, I may well use the transit to Sydney to sort some of these issues out.

When you write about an engine vibration for the scenario, you have the luxury of knowing that it is an engine vibration. In the aircraft, it may not be all that obvious. Taking off, a very likely cause of vibration is a tyre failure. It's always best not to jump to conclusions.

 

[lastrow]

Thank you for your very detailed reply JB. The type of engine vibration I had in mind could be classed as catastrophic, maybe a fan blade parting company or perhaps an engine full of seagulls.
Question - In the early days of under wing jet engines I was told that the engine was mounted onto the wing with four bolts and each bolt was surrounded with a pyrotechnic charge so that if an engine was on fire or other cause that endangered the aircraft the mounting bolts could be severed and the engine released. Fact or fiction?

 

[juddles]

Thank you for your very detailed reply JB. The type of engine vibration I had in mind could be classed as catastrophic, maybe a fan blade parting company or perhaps an engine full of seagulls.
Question - In the early days of under wing jet engines I was told that the engine was mounted onto the wing with four bolts and each bolt was surrounded with a pyrotechnic charge so that if an engine was on fire or other cause that endangered the aircraft the mounting bolts could be severed and the engine released. Fact or fiction?

At the risk of breaking the rules of this thread ( I only pilot 2D devices) I call myth here.

 

[AviatorInsight]

Thank you for your very detailed reply JB. The type of engine vibration I had in mind could be classed as catastrophic, maybe a fan blade parting company or perhaps an engine full of seagulls.
Question - In the early days of under wing jet engines I was told that the engine was mounted onto the wing with four bolts and each bolt was surrounded with a pyrotechnic charge so that if an engine was on fire or other cause that endangered the aircraft the mounting bolts could be severed and the engine released. Fact or fiction?

While it’s true that an engine is only held on by a couple of bolts (only 3 on a 737), I’m not so sure about the fact of having an explosive mounted onto the pylon. I’ve never heard of such a thing. There are vibration isolators in the bolt housings to prevent engine vibration from being transmitted to the airframe. Also the aft mount has only one bolt, but uses a cable to support the rear of the engine if the bolt breaks.

 

[jb747]

Thank you for your very detailed reply JB. The type of engine vibration I had in mind could be classed as catastrophic, maybe a fan blade parting company or perhaps an engine full of seagulls.

I fed an entire flock of galahs through a CF6 once. It kept running, and whilst there was some vibration, you couldn't feel it. The vibration display was showing it, but the engine didn't seem to care.

Question - In the early days of under wing jet engines I was told that the engine was mounted onto the wing with four bolts and each bolt was surrounded with a pyrotechnic charge so that if an engine was on fire or other cause that endangered the aircraft the mounting bolts could be severed and the engine released. Fact or fiction?

Very much fiction. Even the biggest engines are only held on by a couple of very large attachment points. The issues of making one explosively jettison are huge. Not only would 5-10 tonnes of metal be a very unfriendly thing to drop, it would have to have been drop tested to ever be certified...and youtube most certainly does not contain those videos.

Have a look at some of the drop tests, and you'll see why such testing is necessary. To be certified, every possible release configuration would have had to have been tested! Aircraft Store Separation Incidents

 

[mjt57]

I fed an entire flock of galahs through a CF6 once. It kept running, and whilst there was some vibration, you couldn't feel it. The vibration display was showing it, but the engine didn't seem to care.

Yep, turbines, especially steam turbines, can cop a lot of abuse. Our P&W gas turbines had higher vibration limits than the steam ones and they spun nearly twice as fast (5,800 rpm for N2 if I recall).

Do your engines automatically shut down if vibes exceed any specific limits? Or is it manual-matic?

 

[jb747]

Yep, turbines, especially steam turbines, can cop a lot of abuse. Our P&W gas turbines had higher vibration limits than the steam ones and they spun nearly twice as fast (5,800 rpm for N2 if I recall).

Do your engines automatically shut down if vibes exceed any specific limits? Or is it manual-matic?

On some engines, there is an uber overspeed trip for the broken driveshaft case. Otherwise an auto shutdown would be the last thing you want. The vibrating engine might be the best one you have. Pilots don't care about any limits (in some cases anyway)....we simply want the power. Any computerised system that only considered limits would kill people.

 

[JohnM]

T Aircraft Store Separation Incidents

Good grief!

 

[lastrow]

Very much fiction. Even the biggest engines are only held on by a couple of very large attachment points. I must have been thinking of fuse pins, used by Boeing but not Airbus. They are designed to shear off in the event of a wheels up landing thus allowing the engines to break away when they hit the ground.

 

[lastrow]

 

[jb747]

Whilst on the subject of fuse pins, I don’t think they were ever designed to ensure the engine broke off in a gear up landing, but were there to help meet some design requirements with regard to the transmission of loads in the event of failures. Breaking off in a landing is a terrible idea (even though it happens).

They were added to the 747 based on assumptions from 707 incidents. One of these assumptions was than an engine would fall away cleanly...and we now know that this is not the case at all. Two accidents (both freighters) may not have happened at all if Boeing had left these pins out of the design.

Look at the video I referenced above with regard to stores release, and add in some residual power, and you have a classic design for a random outcome.

 

[ChrisGibbs]

Was on Fridays QF442 MEL-SYD A330-3xx service. The APU on the aircraft was U/S and we were relying on ground power. Interesting enough it appears the GPU wasn't able to provide sufficient power and they had to load shed as the power to the seats and various other bits & pieces didn't come to life until we were eventually taxiing. The PIC commented they had to run through a different set of procedures to do an engine start and also ATC through in a runway change for departure prior to pushback.

I understand that without an APU you need a source of high pressure air to do an engine start at the gate. You push back power up and feed the air from the functioning engine to the start the remaining engine/s. What considerations do you need to take into account when doing an engine start without an APU? I would imagine on a 4 engine aircraft it would be a PITA to work through. What considerations do you need to take into account if you have an engine failure on take-off? Does the lack of an APU make things a little more complicated during cruise, descent and landing? For example you would normally notice an engine get shutdown when taxiing to the gate. Without an APU both engines were functioning until the GPU was plugged in at the arrival gate.

 

[jb747]

Was on Fridays QF442 MEL-SYD A330-3xx service. The APU on the aircraft was U/S and we were relying on ground power. Interesting enough it appears the GPU wasn't able to provide sufficient power and they had to load shed as the power to the seats and various other bits & pieces didn't come to life until we were eventually taxiing. The PIC commented they had to run through a different set of procedures to do an engine start and also ATC through in a runway change for departure prior to pushback.

The load shedding is automatic, and as you can tell, starts with passenger service items. I don’ t know how much power an A330 needs, but normally the 380 is plugged in to three external outlets, and we can use four.

Procedurally it depends upon just how much power you have available. In some cases, you’ll keep most of the fuel pumps turned off until after the start. Like everything else, there is a checklist for it.

I understand that without an APU you need a source of high pressure air to do an engine start at the gate. You push back power up and feed the air from the functioning engine to the start the remaining engine/s. What considerations do you need to take into account when doing an engine start without an APU? I would imagine on a 4 engine aircraft it would be a PITA to work through. What considerations do you need to take into account if you have an engine failure on take-off? Does the lack of an APU make things a little more complicated during cruise, descent and landing? For example you would normally notice an engine get shutdown when taxiing to the gate. Without an APU both engines were functioning until the GPU was plugged in at the arrival gate.

Generally you’ll start one engine (in a twin) at the gate, and the other after completing the pushback. You may need slightly more than idle power to get enough air for the cross bleed start.

In the 380, we’ll start two at the gate (which means we’ll have two generators). After the first is running, the others will start via cross bleed with the engines at idle. You need ATC approval for starts at the gate.

In a quad the APU is shut down immediately after start (with the one exception of sometimes wanting to use its air for air conditioning during takeoff). It’s started again just before we get to the gate. It isn’t normally run in flight, and is limited to about FL200 in any case.

The twins will start the APU after any engine malfunction, as that will restore most of the services. In all aircraft, starting the APU gives more electrical redundancy for low vis ops, and can improve the available category (II to III, or IIIA to B).

The APU is not considered for any engine out on take off case. If you need absolute maximum thrust, which you shouldn’t....the simplest solution is to just turn off the packs. You’re at low level, so the lack of pressurisation won’t matter.