Ask The Pilot

[straitman]

[mod hat] Just a reminder of what this thread is about. Reread post #1 if in doubt.

In summary though: As this is an "ask the pilot" thread, we ask that non-pilot members refrain from answering questions that have been directed to pilots until the pilots members have had a good opportunity to answer the question (i.e. at least 7 days). Posts contrary to this request or discussions that get too far off topic may be removed or moved to a more appropriate thread or forum so we can retain order and respect in this thread.

Thanks

[/mod hat]

 

[jb747]

Recently had the pleasure of some holiday flights after a 2 year break, and have a question for any A330-200 experts out there.

On flight from Perth to Melbourne earlier this week, as the aircraft accelerated for take off there was a quite violent vibration / shuddering for a few seconds (felt like 10 seconds, so probably 5) which was rather off putting and enough for my wife to look at me with some alarm. The vibration stopped before leaving the ground and all seemed normal after that.
Now wondering what may have caused it.

Probably ran over a bunch of the centreline lights. People try to miss them, because the rumble is annoying (but harmless). You can miss by either tracking exactly down the centreline, in which case they go between the nose gear tyres, or by offsetting about a metre to either side.

 

[Quickstatus]

Probably ran over a bunch of the centreline lights.

Could the following also cause vibration at takeoff?

irregular tyre wear or wheel imbalance?.
undercarriage retraction

 

[jb747]

Could the following also cause vibration at takeoff?

irregular tyre wear or wheel imbalance?.
undercarriage retraction

I assumed from the OP's comment that they were on the ground and the vibration started, and then went away, whilst still on the runway. The lights are the only thing that make sense for that...that I can think of.

Tyre wear or imbalance tends to be absorbed into all of the other things going on. Even a tyre failure isn't necessarily all that noticeable inside the aircraft. And if the vibration were from a tyre, it would continue to get worse until retracted.

During the retraction sequence the main gear brakes are automatically applied. I don't recall feeling this. But, the nose gear, being unbraked can give vibration for a short period as it retracts.

 

[Quickstatus]

e nose gear, being unbraked

Is that by design? - ie better unbraked vs braked?

 

[straitman]

Is that by design? - ie better unbraked vs braked?

No aircraft that I am aware of have nose wheel braking.

 

[Quickstatus]

No aircraft that I am aware of have nose wheel braking.

Why is that?
More directional stability in a tri carriage configuration with rear bias braking

Or doesn’t significantly improve braking distance?

 

[AviatorInsight]

Is that by design? - ie better unbraked vs braked?

A really good question! Yes it's intentional that most (some 727s and Convair 660s? had them) commercial aircraft don't have nose gear brakes. The center of gravity doesn't move on landing. Because the main wheels are located closer to the CG then most of the braking effect will be around that area.

From what I've read on the aircraft that had them, they had very little effect of braking and not to mention the cost of maintaining them as well. One source mentioned that for the 727, once the main wheels had achieved 60% of the braking action then the nose gear brakes would become effective.

 

[ayebee]

Probably ran over a bunch of the centreline lights. People try to miss them, because the rumble is annoying (but harmless). You can miss by either tracking exactly down the centreline, in which case they go between the nose gear tyres, or by offsetting about a metre to either side.

Thanks for the reply.
I have heard the "muted bangs" through the nose wheel shocks of runway lights, but TBH this was enough to feel and hear the cabin shaking. Maybe some form of resonance ?

 

[jb747]

Why is that?
More directional stability in a tri carriage configuration with rear bias braking

Or doesn’t significantly improve braking distance?

The nose gear’s main job is to stop the nose dragging on the ground. Even at low speeds, you have to be very careful with steering inputs, as it simply has very little grip, and it’s extremely easy to make it skid, even on a dry taxiway. If you added braking to its functions, you’d reduce the already minimal steering, so I’m sure the benefits/negatives don’t come down in favour braking. I can’t imagine it doing much other than cycling the ABS if you had braking, given how little weight is upon it for much of the time. In a 570 tonne A380 there was something in the order of 10 tonnes on the nose. A trivial portion.

 

[Saab34]

Lights Off or On at night in the coughpit? Transcon vs MEL-SYD would you take the same approach? Is it easy to doze off in the dark coughpit?

JB, which coughpit has better lighting? AB or B?

 

[jb747]

Lights Off or On at night in the coughpit? Transcon vs MEL-SYD would you take the same approach?

I was mostly lights off.

Is it easy to doze off in the dark coughpit?

I don’t think it makes much difference.

JB, which coughpit has better lighting? AB or B?

A bit of a mixed bag. They both did some parts well, and about equally badly.

 

[mjt57]

In a 570 tonne A380 there was something in the order of 10 tonnes on the nose. A trivial portion.

Sounds like it's better balanced than my caravan....

 

[jb747]

I'll add to the above. The replies tend to be short when I'm on an iPad without keyboard.

Some people did think that keeping the lights on at night helped to keep them awake. I took the opposite view, as it blocked out the outside view...but then some don't look outside all that much. Once your eyes became accustomed to the darkness, it was surprising how much you could see, even on a moonless night. Not all cloud shows up well on radar, and flashes from lightning could be visible well over the horizon, and long before the radar spotted it. Other aircraft, features on the ground, and sky, meant that it wasn't just a featureless black void.

 

[Quickstatus]

keeping the lights on at night helped to keep them awake

Correct it does not. Circadian rhythms are very powerful.

featureless black void.

What about the horizon at night?

(Typos corrected)

 

[straitman]

What horizon?
It can vary between quite good on a clear star and moonlit night to nothing if it’s overcast.

 

[jb747]

What horizon?
It can vary between quite good on a clear star and moonlit night to nothing if it’s overcast.

There's a trap with faint horizons too. It may not be the actual horizon, but anything from a coastline to a bank of cloud. And if it's at an angle it can start to reset your internal horizon to the incorrect one. That's one of the reasons why what you feel is NEVER part of how you fly on instruments.

 

[aaa99]

Sorry if this is a bit basic, but I have a question about the function of APU upon landing and why it is not possible to de-board the aircraft until ground power is established.

Was on NZ123 on Sunday flying from AKL-MEL, a 777 (ZK-OKQ). Upon landing in Melbourne on time, we reached our gate okay. But then we were told that the APU had failed. So, we had to wait for Ground power unit. As there were a lot of aircraft taking off at this time, we were told, we might have to wait 20mins. After 20 minutes, the first GPU came around and it also failed. Then we waited for another one (which failed) and another one. Finally, we were out after sitting 1.5hours at the gate. Flightradar24 and other tracking websites dont show this delay, but you can see that the outbound NZ124 on the day was delayed by 2+ hours. The whole time, one of the engines was running and providing power to the aircraft. So no lights out or anything.

I couldnt actually tell if it was more that gate couldnt be powered as a result. I was on the right and couldn't really see what was going on on the other side. If so, is it not possible to power the gate via the engine power safely?

 

[747sp]

I was wondering with wake turbulence separation and the Concorde was there more turbulence near the ground compared to subsonic plane of a similar size?

 

[jb747]

Sorry if this is a bit basic, but I have a question about the function of APU upon landing and why it is not possible to de-board the aircraft until ground power is established.

The APU provides electricity and pressurised air for the airconditioning. If you don't have engine, APU, or GPU power, it will be very dark in the cabin. There's no battery backup to the normal lighting. There would be emergency lighting only, so I can see why they'd be reluctant to have people moving.

Was on NZ123 on Sunday flying from AKL-MEL, a 777 (ZK-OKQ). Upon landing in Melbourne on time, we reached our gate okay. But then we were told that the APU had failed. So, we had to wait for Ground power unit. As there were a lot of aircraft taking off at this time, we were told, we might have to wait 20mins. After 20 minutes, the first GPU came around and it also failed. Then we waited for another one (which failed) and another one. Finally, we were out after sitting 1.5hours at the gate.

You've got especially good luck with GPUs!

The whole time, one of the engines was running and providing power to the aircraft. So no lights out or anything.

And that's the reason. You'd need to shut the engines down before any ground equipment (i.e. the bridge) would be allowed near the aircraft. But shut that engine down, any you'll be plunged into the dark.

I couldnt actually tell if it was more that gate couldnt be powered as a result. I was on the right and couldn't really see what was going on on the other side. If so, is it not possible to power the gate via the engine power safely?

Gates/aerobridges are powered by the terminal. They may be used to feed power to the aircraft, but not vice versa. But they cannot be moved on to the aircraft whilst any engines are still running.

The aircraft doors can be operated normally, with, or without, aircaft power.