One of our pilots answers common myths and questions asked by passengers

What does it sound like a dog is barking underneath the floor of the aircraft after engine start?

Ah, this is question we are often asked. I am not an Airbus expert (I fly the jumbo) but this is the noise of the PTU (Power Transfer Unit) its part of the hydraulic system and the PTU activates when hydraulic system pressure falls below a certain level. This is normally when Airlines are using single-engine operations on the ground, although the PTU can continue sometimes after both engines have been started.

Most airlines taxi out and in on one engine to save fuel and engine wear. This noise only occurs on twin engine Airbus aircraft but the noise is very noticeable and it does sound like your neighbours dog is trapped in the hold!

Rest assured this is normal and will stop when the second engine is running as the PTU is a backup redundancy based system.

Why do the engines sound like they are throttling back or shutting down shortly after take off?

There are various segments to a take off defined in the aviation module of performance.

Without going into too much technical jargon, effectively an aircraft does not require the amount of thrust it needs for take-off to the initial climb phase.

There are other benefits too, engine life is extended by not operating the engines for any significant period close to maximum thrust and by reducing thrust, the aircraft has less impact on the environment in terms of noise (for residents in the ground and passengers in the aircraft) and pollution.

Some airports have strict noise regulations and will enforce what are called noise abatement procedures on take-off.

The procedures can vary depending on where the noise issues are – one procedure is to get away from the ground with max thrust as quickly as possible then reduce thrust and accelerate the aircraft, this is typically used if there are noise sensitive areas further away from the airport and the other one is to reduce thrust as soon as possible but climb away at a lower speed to get the aircraft away from a noise sensitive area near the airport as quickly as possible.

Most take-off’s will involve some sort of reduction of thrust at 1000ft (20-30 seconds after take-off) and that is the change in the engine noise that you will notice. It is more pronounced on some aircraft engines than others particularly if it was what we call a performance limiting take off.

This is where the aircraft has to use maximum thrust to take off and climb away safely from the airport this could be due to a heavy aircraft (full passengers and bags / freight) and / or a short runway, high elevation airfield or high temperatures and less than favourable wind direction.

If this is the case the range from max thrust to climb thrust is greater therefore you get a more pronounced sound. Passengers worry about this thrust reduction especially if you have watched the film ‘Sully’ , but again it is perfectly normal.

What are the reasons for the aircraft to take off again when coming into land?

We call these procedures go-arounds or missed approaches.

Although it will almost certainly catch the passengers by surprise there is nothing to worry about, in fact it is normally the safer option which is why it has been chosen by the flight crew.

There are many reasons for go-arounds, but the main one is following an instruction from ATC, this would be for another aircraft not clearing the runway in time which is the most common cause of a go-around, it could also be for a bird strike on the runway from a previous aircraft.

Airlines have what we call stable approach criteria. This means that at a certain point on the approach normally 500ft or 1000ft the crew have to meet certain parameters which would include being in the landing configuration (gear down and locked and flaps in the planned landing setting), on the correct approach path (cannot be low or high) and at the correct speed with some leeway for windy days.

If these parameters are not met then the flight crew must elect to discontinue the approach and go-around.

Other reasons for go-arounds are weather, it may become apparent that the weather is outside the limits of the aircraft, i.e too windy, cloudbase too low, too much fog or the runway is too much contaminated by water / snow or ice.

Crews may also elect to go-around at any time if they are not satisfied with the approach or they may be landing the aircraft outside of the touchdown zone which could mean there would not be enough runway to slow down and stop.

A go-around can happen at any point including after the wheels have touched down.

Go-arounds do happen and they are nothing to be worried about. 9 times out of 10 they will re-position on the approach and land within 10 minutes.

When does an aircraft configure for landing?

It can be a fairly long process configuring an aircraft for landing.

To bring an aircraft in to land at a safe speed where we can stop the aircraft we have to use flaps and slats to allow us to fly at a slower speed, these devices are either operated electrically or hydraulically depending on aircraft type and it can take a while to extend these huge surfaces.

Descent planning is crucial, flight crews have to plan how many miles they require to lose the cruising altitude they are at and then slow the aircraft down.

Most aircraft are not very good at slowing down and descending at the same time as they are built to fly efficiently.

There may be restrictions in terms of speed and heights from ATC. Typically at 10000ft you have to reduce speed from your initial descent speed of 290-310kts (345mph) to 250kts.

We normally plan for a 3 degree angle descent so we can use our 3 times table to roughly work out how many miles we require to descend.

For example if we are at 38000ft which is a typical cruising altitude then 38 x 3 is 114 nautical miles and depending on the aircraft you would need another 10 miles to slow down so 125 miles before the airport we would need to descend at the very latest.

The point at which we would start configuring the aircraft would vary on the day depending on aircraft type, ATC and the airport we are flying into.

Typically at 15 miles we would reduce from 250kts to 210kts and start lowering flaps and slats. 180kts and an intermediate flap setting is pretty normal at 10 miles. Gear would be selected down at around 6 miles and then final flap selected a final approach speed acquired.

It is easy to get too high on the approach this may be due to wind or ATC restrictions.

We have speedbrakes that we can use which help us effectively ‘dump lift’ so we can use these to increase the rate of our descent or slow down.

These are located on the upper part of each wing and you can always feel when they are deployed as you feel a slight rumble throughout the cabin a little bit like turbulence.

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