airmarshal
ELITE MEMBER
- Joined
- Jul 28, 2010
- Messages
- 9,354
- Reaction score
- 11
- Country
- Location
Monarch Blog | What is turbulence? Captain Nigel Webster explains
What is turbulence? Captain Nigel Webster explains
May 20, 2015
3 562
Turbulence is not a very pleasant feature of air travel, and causes understandable anxiety to some of our passengers. It results from rapid changes of wind direction and strength, which in turn make the aircraft accelerate or decelerate, bank or move up and down or from side to side. In fact we usually experience a combination of several of movements which can do uncomfortable things to your stomach. However, I hope it will comfort you to know that aircraft are designed to withstand turbulence safely. And of course, pilots are trained to avoid turbulence and to reduce the effects of it where it is unexpected or unavoidable.
A civil aircraft is designed to be naturally stable, so if it is suddenly disturbed into diverging from the path it was quite happily taking in the previously smooth stream of air, it will try to return to its original path without pilot intervention. During most phases of flight – other than take-off and final approach – the autopilot is usually flying the aircraft, and will cope with this process very smoothly. The autopilot also normally has control of the engines, so there may be changes in engine noise – which is more noticeable in the rear cabin, behind the engine exhausts. (Incidentally, when we fly over or under a large, four-engined aircraft like a B747 or an A380, separated from us by 1000ft, we can briefly hear the roar of their engines. I like to think they can hear our two as well – but I digress…)
If you’re looking out the window, you’ll see that wings and engine pylons will flex visibly during turbulence – they are designed to do this, so don’t worry! All civil aircraft, including ours, are designed to withstand loads of +2.5 g to -1.0 g (and considerably more than that in practice), which is more than enough to cater for even extreme turbulence. Our Airbus fly by wire (FBW) aircraft have a load alleviation function, which senses the start of a big bump and automatically deflects the flying controls to mitigate it. The FBW system also limits the loading the autopilot can apply, which can be a further safety enhancement.
The main sensation you will feel is the aircraft climbing or descending – being pressed slightly into your seat, or pulled out of it, firstly by the original disturbance, and then by the aircraft reacting to it. The airstream at this stage is however likely to be making multiple changes in direction or speed, so the autopilot will need to make multiple reactions. These changes are felt most at either end of the aircraft, but they are more marked in the tail: when the aircraft is pulling up, those at the front of the cabin – which includes us pilots of course – are pressed slightly into our seats, which feels natural.
Those at the rear will have the double sensation of being pulled out of their seats – as the tail goes down to start the climb – followed by being pressed back into the seat as the climb starts. It follows from this that over the wing is the most comfortable place to be (because the effect of the controls pitching the aircraft up or down is minimised). Our flying speed – typically around 520 mph in the cruise, or a mile every seven seconds – makes every little bump feel worse (think driving over a pothole at 70 mph on the motorway). The aircraft rarely climbs or descends more than 50 ft or so – which is insignificant.
So that is what is going on after the aircraft has been so rudely interrupted whilst going about its intended business – but why was it interrupted in the first place? What causes these rapid changes of wind strength and direction, and why don’t we simply avoid them?
There are three main types of turbulence. The first type is caused by a physical feature, like a mountain or a tall building close to a runway. A strong wind being forced around this obstacle will result in turbulence downwind of the object. Examples are Gibraltar, where history and geography dictate that the runway lies very close to the 1400 ft rock – a strong wind blows turbulence across the approach, and we can sometimes even see the turbulence ruffling the water there as we come in to land. Approaching the north-easterly runway in Geneva brings us close to a ridge, and there is often a short burst of turbulence as we pass. And a strong south-westerly wind makes for a bumpy final approach into Gatwick, because of a large hangar just south of the runway. (Pilots call this ‘Freddie’s Revenge’, after the original owner Freddie Laker, whose airline was driven out of business in the 1980s.) If we need to fly from an airport that suffers from this sort of turbulence, we have to accept it – but on the bright side it will be well understood, and forecast.
A second type of turbulence is associated with cloud. Cloud is formed by unstable, moist air rising faster than the surrounding air, until the moisture condenses and it eventually becomes stable and stops rising. Enter any cloud and there may be turbulence caused by this rising, unstable air: the higher up or faster we are, the more we are likely to be bounced around. Even the thin sheets of cloud we sometimes encounter during the cruise can cause a modest but annoying little ripple. If possible we will climb or descend to find more comfortable air. Broadly speaking, the higher up we are the fewer clouds there will be, and the easier they will be to avoid.
Turbulence is more marked in cumulus clouds – the cauliflower-shaped ones, where as we approach them we can see the cloud boiling upwards. In crowded airspace we often need to pass through small ones, with the resultant brief and mild bump. Their much bigger (and thankfully rarer) relations are cumulonimbus clouds – thunderstorms. We do not enter these, and maintain a minimum vertical and horizontal distance from them, because they consist of unstable air that is moving up (and sometimes down) very rapidly. Thunderstorms are associated with heat, often with warm water and sometimes with high ground too, so the most powerful ones are prevalent over the eastern USA, Africa and the Caribbean and Indian oceans – not on Monarch’s scheduled routes. Nevertheless, thunderstorms can occur practically anywhere – particularly the eastern Mediterranean and the Balkans in late summer.
We have radar which detects the movement of air – actually by detecting the large water droplets being whizzed around inside the cloud. The radar display is colour coded – green, yellow and red for the worst bits, and the shape of these contours gives us further information about what to avoid, and which way to turn. Air Traffic Control allows us to deviate around them, so you (and I) will not fly in one, although they do spawn cumulus in their immediate area, which can be bumpy; sometimes these have to be entered briefly during approach or departure. If you have experienced being comprehensively shaken around in an aircraft inside cloud, it was probably from the cloud near a cumulonimbus storm, rather than the core of the storm itself. Because such clouds are isolated, the turbulence they bring is thankfully short-lived, as we fly away from and around them.
The final type of turbulence is clear air turbulence (CAT). This can be more widespread, but is also reasonably predictable – on every flight we have a chart which predicts where CAT might be. Sometimes there is no obvious reason other than differences in the layers of the local air mass: but it is frequently associated with jet streams (jets): rope-like cores of fast-moving air encircling the globe.
We always expect a 120 mph westerly jet across North Africa and Saudi Arabia, and the polar jet zigzags across the Atlantic to bring the UK its quirky, variable weather. (It slipped south over Northern France to bring a cooler, changeable May bank holiday, and doubtless will return north to warm things up as you return to work.)
Strong winds do not necessarily mean turbulence, but it is often found where the jet turns a corner, or its speed reduces. Why? Imagine you are in a London Underground station in rush hour. As the crowd charges down the corridor into a right-angled turn, those on the inside of the turn in particular have to slow and deviate a bit to avoid collision with the stranger in front. After the turn the crowd resumes its original pace. Air molecules behave exactly the same way – particularly in a slowing stream of air. A change in altitude can improve things, but sometimes we just have to sit it out until we have flown into a different air mass.
With the seat-belt sign on the cabin crew may continue to walk around. They are trained and experienced in the feel of turbulence, and there are fewer of them. They will have checked with us whether it is safe to move around the cabin, and know what to do if things suddenly deteriorate: a toilet queue would fare rather worse. We will have our seat-belts fastened – as pilots do at all stages of flight, smooth or not, even as passengers. We are not enjoying the ride any more than you are, but we are armed with information from our charts, our radar, and sometimes other aircraft and Air Traffic Control. You can help us out by heeding cabin crew instructions, allowing us to concentrate on assessing how we can return to more comfortable flying conditions.
So I hope that gives you a feel for what is going on. We avoid, or mitigate, turbulence as best we can – after all, you might fly a couple of times a year. We are doing this several times a week!
And finally, here’s a thought to bear in mind: the most dangerous thing about flying is the drive to the airport.
So, in summary:
1. Turbulence can be unpleasant but flying through it is normal and safe
2. The most comfortable place to sit in the cabin is over the forward part of the wing, for the smoothest ride
3. Turbulent air can be caused by physical features on land, clouds and in clear air
4. Some types of turbulence are predictable and avoidable, others are not
5. Aircraft are built to withstand much greater forces than normal turbulence causes
6. Pilots are highly experienced at identifying and avoiding the worst kinds and causes of turbulence and work hard to mitigate or avoid it as much as they can
What is turbulence? Captain Nigel Webster explains
May 20, 2015
3 562
Turbulence is not a very pleasant feature of air travel, and causes understandable anxiety to some of our passengers. It results from rapid changes of wind direction and strength, which in turn make the aircraft accelerate or decelerate, bank or move up and down or from side to side. In fact we usually experience a combination of several of movements which can do uncomfortable things to your stomach. However, I hope it will comfort you to know that aircraft are designed to withstand turbulence safely. And of course, pilots are trained to avoid turbulence and to reduce the effects of it where it is unexpected or unavoidable.
A civil aircraft is designed to be naturally stable, so if it is suddenly disturbed into diverging from the path it was quite happily taking in the previously smooth stream of air, it will try to return to its original path without pilot intervention. During most phases of flight – other than take-off and final approach – the autopilot is usually flying the aircraft, and will cope with this process very smoothly. The autopilot also normally has control of the engines, so there may be changes in engine noise – which is more noticeable in the rear cabin, behind the engine exhausts. (Incidentally, when we fly over or under a large, four-engined aircraft like a B747 or an A380, separated from us by 1000ft, we can briefly hear the roar of their engines. I like to think they can hear our two as well – but I digress…)
If you’re looking out the window, you’ll see that wings and engine pylons will flex visibly during turbulence – they are designed to do this, so don’t worry! All civil aircraft, including ours, are designed to withstand loads of +2.5 g to -1.0 g (and considerably more than that in practice), which is more than enough to cater for even extreme turbulence. Our Airbus fly by wire (FBW) aircraft have a load alleviation function, which senses the start of a big bump and automatically deflects the flying controls to mitigate it. The FBW system also limits the loading the autopilot can apply, which can be a further safety enhancement.
The main sensation you will feel is the aircraft climbing or descending – being pressed slightly into your seat, or pulled out of it, firstly by the original disturbance, and then by the aircraft reacting to it. The airstream at this stage is however likely to be making multiple changes in direction or speed, so the autopilot will need to make multiple reactions. These changes are felt most at either end of the aircraft, but they are more marked in the tail: when the aircraft is pulling up, those at the front of the cabin – which includes us pilots of course – are pressed slightly into our seats, which feels natural.
Those at the rear will have the double sensation of being pulled out of their seats – as the tail goes down to start the climb – followed by being pressed back into the seat as the climb starts. It follows from this that over the wing is the most comfortable place to be (because the effect of the controls pitching the aircraft up or down is minimised). Our flying speed – typically around 520 mph in the cruise, or a mile every seven seconds – makes every little bump feel worse (think driving over a pothole at 70 mph on the motorway). The aircraft rarely climbs or descends more than 50 ft or so – which is insignificant.
So that is what is going on after the aircraft has been so rudely interrupted whilst going about its intended business – but why was it interrupted in the first place? What causes these rapid changes of wind strength and direction, and why don’t we simply avoid them?
There are three main types of turbulence. The first type is caused by a physical feature, like a mountain or a tall building close to a runway. A strong wind being forced around this obstacle will result in turbulence downwind of the object. Examples are Gibraltar, where history and geography dictate that the runway lies very close to the 1400 ft rock – a strong wind blows turbulence across the approach, and we can sometimes even see the turbulence ruffling the water there as we come in to land. Approaching the north-easterly runway in Geneva brings us close to a ridge, and there is often a short burst of turbulence as we pass. And a strong south-westerly wind makes for a bumpy final approach into Gatwick, because of a large hangar just south of the runway. (Pilots call this ‘Freddie’s Revenge’, after the original owner Freddie Laker, whose airline was driven out of business in the 1980s.) If we need to fly from an airport that suffers from this sort of turbulence, we have to accept it – but on the bright side it will be well understood, and forecast.
A second type of turbulence is associated with cloud. Cloud is formed by unstable, moist air rising faster than the surrounding air, until the moisture condenses and it eventually becomes stable and stops rising. Enter any cloud and there may be turbulence caused by this rising, unstable air: the higher up or faster we are, the more we are likely to be bounced around. Even the thin sheets of cloud we sometimes encounter during the cruise can cause a modest but annoying little ripple. If possible we will climb or descend to find more comfortable air. Broadly speaking, the higher up we are the fewer clouds there will be, and the easier they will be to avoid.
Turbulence is more marked in cumulus clouds – the cauliflower-shaped ones, where as we approach them we can see the cloud boiling upwards. In crowded airspace we often need to pass through small ones, with the resultant brief and mild bump. Their much bigger (and thankfully rarer) relations are cumulonimbus clouds – thunderstorms. We do not enter these, and maintain a minimum vertical and horizontal distance from them, because they consist of unstable air that is moving up (and sometimes down) very rapidly. Thunderstorms are associated with heat, often with warm water and sometimes with high ground too, so the most powerful ones are prevalent over the eastern USA, Africa and the Caribbean and Indian oceans – not on Monarch’s scheduled routes. Nevertheless, thunderstorms can occur practically anywhere – particularly the eastern Mediterranean and the Balkans in late summer.
We have radar which detects the movement of air – actually by detecting the large water droplets being whizzed around inside the cloud. The radar display is colour coded – green, yellow and red for the worst bits, and the shape of these contours gives us further information about what to avoid, and which way to turn. Air Traffic Control allows us to deviate around them, so you (and I) will not fly in one, although they do spawn cumulus in their immediate area, which can be bumpy; sometimes these have to be entered briefly during approach or departure. If you have experienced being comprehensively shaken around in an aircraft inside cloud, it was probably from the cloud near a cumulonimbus storm, rather than the core of the storm itself. Because such clouds are isolated, the turbulence they bring is thankfully short-lived, as we fly away from and around them.
The final type of turbulence is clear air turbulence (CAT). This can be more widespread, but is also reasonably predictable – on every flight we have a chart which predicts where CAT might be. Sometimes there is no obvious reason other than differences in the layers of the local air mass: but it is frequently associated with jet streams (jets): rope-like cores of fast-moving air encircling the globe.
We always expect a 120 mph westerly jet across North Africa and Saudi Arabia, and the polar jet zigzags across the Atlantic to bring the UK its quirky, variable weather. (It slipped south over Northern France to bring a cooler, changeable May bank holiday, and doubtless will return north to warm things up as you return to work.)
Strong winds do not necessarily mean turbulence, but it is often found where the jet turns a corner, or its speed reduces. Why? Imagine you are in a London Underground station in rush hour. As the crowd charges down the corridor into a right-angled turn, those on the inside of the turn in particular have to slow and deviate a bit to avoid collision with the stranger in front. After the turn the crowd resumes its original pace. Air molecules behave exactly the same way – particularly in a slowing stream of air. A change in altitude can improve things, but sometimes we just have to sit it out until we have flown into a different air mass.
With the seat-belt sign on the cabin crew may continue to walk around. They are trained and experienced in the feel of turbulence, and there are fewer of them. They will have checked with us whether it is safe to move around the cabin, and know what to do if things suddenly deteriorate: a toilet queue would fare rather worse. We will have our seat-belts fastened – as pilots do at all stages of flight, smooth or not, even as passengers. We are not enjoying the ride any more than you are, but we are armed with information from our charts, our radar, and sometimes other aircraft and Air Traffic Control. You can help us out by heeding cabin crew instructions, allowing us to concentrate on assessing how we can return to more comfortable flying conditions.
So I hope that gives you a feel for what is going on. We avoid, or mitigate, turbulence as best we can – after all, you might fly a couple of times a year. We are doing this several times a week!
And finally, here’s a thought to bear in mind: the most dangerous thing about flying is the drive to the airport.
So, in summary:
1. Turbulence can be unpleasant but flying through it is normal and safe
2. The most comfortable place to sit in the cabin is over the forward part of the wing, for the smoothest ride
3. Turbulent air can be caused by physical features on land, clouds and in clear air
4. Some types of turbulence are predictable and avoidable, others are not
5. Aircraft are built to withstand much greater forces than normal turbulence causes
6. Pilots are highly experienced at identifying and avoiding the worst kinds and causes of turbulence and work hard to mitigate or avoid it as much as they can
. My knowledge comes mostly from fear of flying sources. However, several articles have referenced the 787 and 777 as including lateral and vertical gust suppression by as much as 60 to 70%.
