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wpm1000
7th Aug 2007, 12:59
I travel regularly (2/3 times per week) in the Gulf, and have used most of the Gulf carriers, but being based in Bahrain, it is typically with Gulf Air. Now I know that the problems of GF have been well aired on this site but I'd be grateful for anyone with the tech know how to help with the following. On the GF a320, take off in all weather, even with few pax aboard and at all times of the day or night seems to take much longer than I've experienced on other airlines - I'm talking about accelaration time on the ground before lift off. On recent flights on different a/c (a320), at the moment of takeoff there has been a pronounced and very audible shiver which seems to run through the cabin from front to rear as the plane pulls up. Any thoughts on what this is or whether it is anything to be concerned about? Many thanks.

G-BPED
11th Aug 2007, 13:25
Hello wpm1000,

First of all, I am not a commercial pilot and as such have no in depth technical knowledge of the Airbus or any other commercial jet.

However, being a frequent traveller who is interested in aircraft I have spoken to pilots in the past and asking the same question about the length of take off roll on an A320 compared with say a B757 or B767 I can tell you this.

I understand that runway length, loading ( fuel, pax etc ) weather and other factors are taken in to account and the engine power is adjusted accordingly. I believe the computer on board the "Bus" calculates the engine power required. The aircraft appears to accelerate a bit slower and thus the take off roll is longer as full power ( 100% N1 I think ) is not always used.

A B757 always seems very eager to get into the air whereas an Airbus uses more runway.

As for the "shiver" I would say that is more like an oscillation and I too have noticed it but, I have no idea what that might be and if any pilot is looking at the SLF forum postings maybe they can answer.

Regards G-GBPED ( Now waits to be corrected by an Airbus expert ) ;)

Dan Winterland
12th Aug 2007, 00:55
Aircraft very rarely take off at full power. The redution in power can be a 'de-rate' of the engine, (telling the performance software the engine has less power (or in some cases that it's a different version of the engine with less power), or it can be with an 'assumed' temperature where a different outside air temperature is uded to acheive the thrust reduction. The benefit of this is that engine life is prolonged and savings are made. It sounds daft at first, but a good analogy is if you think when you accelerte your car, you could use full accelerator but probably don't to preserve the life of the engine.

The parameters which determine how much power reduction is safe are runway length, tempearture, wind, air pressure and runway condition (i.e wet or icy) and condition of the aircraft - some failures (a brake unit for example) can still allow a takeoff to be made. Bahrein for example has a very long runway, so quite a big reduction can be made which is what you have experienced. The maximum power reduction allowed by licensing authorities is 25%.

The calculations are done either from a book of tables or graphs, or from the aircraft's own flight management computer software, or in my airline's case, from a datalink to company headquaters. Interestingly, the Airbus (for all it's complexity) doesn't have the take off performance built in. Boeings tend to (depending on the customer's requirements) but it wasn't permitted to use it by our licenscing authority on the one Boeing type I used to fly.

I should mention that it's perfectly safe. The calculation assumes that if an engine fails on take off, the remaining good engine can safely get the aircraft airborne with the reuduced power. The pilot of course has the option of using full power for increased safety.



The 'shiver' on take off could be one of two things. Immediately on take off, a small thump is felt as the oleos (the aircraft's suspension which are built into the gear legs and is based on compressing oil and gas, a bit like a shock absorber on a car) reach their extension limits. Also, the wheels are braked as you don't want a fast rotaing heavy wheel spinning when it's retracted ito it's wheel well. The mainwheels have the main brakes to stop them, but there is no brake on the nosewheel. So when it retracts, it rubs on a brakepad attached to the top of the wheel well. It is probably this rubbing (which lasts for several seconds) which you can sense.

TightSlot
12th Aug 2007, 08:25
Excellent answer Dan - Thanks.

As one whose jumpseat sits immediately over the nosewheel on 757, I can testify first-hand to the noise and vibration experienced after take-off when the wheel hits the rubber snubbers. It is not uncommon for me to have to explain and reassure the more nervous passengers in the area.

G-BPED
12th Aug 2007, 10:37
Hello Dan W,

Thanks for your detailed information, certainly much better explained than my attempt.

I hope WPM views your response.

Regards,

G-BPED

wpm1000
12th Aug 2007, 13:18
Thanks guys, that's very helpful and informative. I'll try and "switch off" in future and not get too distracted by every bump and bang!