It's called the stick shaker – an 'attention-getter' for pilots to tell them that their aircraft is about to stall.
And it's a warning that one serving airline captain received as he flew over north Africa in a fully laden Boeing 747 a few years ago.
The cause, he says, was extreme turbulence, which had pushed the aircraft to below its minimum speed, despite the engines being on full power.
He's re-living the dramatic incident with MailOnline Travel in a chat about turbulence he is keen to have in a bid to help ease passengers' nerves about the phenomenon.
He explains why we're so scared of it, how pilots manouevre around it and what they do when it stalls their plane.
The UK-based pilot, speaking anonymously, explains that there are two types of turbulence - one that can be spotted, and another, more unnerving variety.
He says: 'There's turbulence associated with precipitation, be it rain or snow, and we can identify that on a weather radar that we have on the aircraft.
'So we can always avoid that type of turbulence. Unless we need to come in to land in that sort of turbulence, in which case it's a judgement call about whether the turbulence is within safe limits. If it is, we'll commit to an approach and have a go. If it's not within safe limits we'll divert to another airfield, where it is safe, or hold off until such time as it is safe to land.
'The sort of turbulence that people often get anxious about is clear air turbulence, which is often what happens at high altitude. And that's associated with the intercontinental jet streams that circle the globe. And where these jet streams collide, obviously is determined by weather patterns. And when they collide it creates ripples in the air. And that's what clear air turbulence is.
'It's quite easily predictable in terms of where it might be on a given day, and weather forecasters are really good at forecasting it, such that pilots can avoid the worst areas of it, but we can't see it on radar, so we tend to rely on other aircraft reporting it, such that we can either avoid it, or put the seat belt signs on.'
This lurking, unseen turbulence, is why it's always a good idea to belt up on an aircraft.
He continues: 'Because we have a lot of people now who fly an awful lot, sometimes they're a bit casual about whether they should return to their seats and put their seatbelts on, and people think it's a bit of a drag, but there will be that one time in a hundred when it suddenly becomes really important to be in your seat.'
It's at this point that the captain decides to recount his terrifying turbulence-over-Africa story.
He says: 'I had a situation going over north Africa in a 747 about 10 years ago where it went from as calm and smooth as we are now, in MailOnline's offices, to a minute later we had the seatbelt signs on and we needed people in their seats. And a minute after that we made a PA announcement to say we needed cabin crew back in their seats and a minute after that the wind had swung round enough that the stall warning went off on the aeroplane because the wind had shifted so quickly that we'd actually gone below our minimum speed.
'So we had a stall warning and what's called a stick-shaker, which is the attention-getter for pilots to say a stall is imminent.
'So we're in a fully laden 747, with all four engines at full power, and we still couldn't maintain altitude. So, while that event is unusual, it is possible for things like that to happen. And the last thing you need, in that situation, is to think that people are still running around in the back.
'So that's why we always take the conservative side of things, and put the seatbelt signs on early, just in case things evolve into a situation that whilst it's not dangerous, it's certainly not ideal to have people running around because the aircraft did move around quite significantly in that event.
'It only lasted about three or four minutes and after that it was fine.'
The pilot explains that he came out of the stall by pointing the plane downwards.
He says: 'You push the nose forward, keep the power on, let the aircraft accelerate. Aircraft like flying, they don't like falling out of the sky, and you've got to try pretty hard to make them do that. So, just push the nose forward, accelerate the aeroplane, and we return to our assigned altitude.
'For two to three minutes it was exciting, it was proper flying, the autopilot wasn't coping very well with it, so you take the autopilot out with a little push button on the control column and you go back to basic flying skills that keep the aircraft safe. That's why we're there.
'That's why there's always two of us on the flight deck and why we take the business of flight safety very seriously. And part of that is putting the seatbelt signs on.'
It turns out that even though they work for rival carriers, pilots are very good about radioing each other with turbulence news.
The pilot says: 'There's no favouritism or protocol. So if an Air France aeroplane is in some turbulence and it's got a rival aeroplane behind it, they'll let us know. There's a good deal of respect between pilot communities worldwide because that's how we keep aviation as safe as it possibly can be.'
The pilot believes that a fear of turbulence – and flying in general – is down to a feeling of a lack of control.
He says: 'We live in a 24/7 environment where you can do anything you want, anytime you want, with whoever you want. And then you get to the airport, they make you check your bag in, if it's too big you get told off, you get to security and you have to take your shoes off, you have to take your belt off, give up your laptop, you can't take any toothpaste because it might be a bomb.
'All that stuff happens and it just reminds people that they're not in control anymore. You then get told where you're going to sit, you get told when you can sit down and stand up, and that strips away lots of layers of this illusory layers of control that people have.
'People think they can do whatever they want whenever they like and aviation takes that away.
'And often the reaction that goes with that is not a brilliant one. And that's the reason people get anxious, I think.'
'But a lot of this stuff isn't always rational. We talk a lot in aviation training about the high road and the low road now.
'So you've got the hypothalamus, where all the brainy stuff happens and the low road where, which is effectively the inner-chimp. So, if I go to pick something up and it's hot I don't say "oh, that's hot, I'll take my hand away". The inner chimp does it for me. The low road does it immediately. And what you have to do as a pilot is you have to separate the low-road startle factor where the chimp wants to run and hide, or wants to try and batter the controls, with the high road "ok, I'm going to let my training take over now, let's be calm, take a deep breath and analyse what's going on".
'And having to do that is not always easy. But as a pilot you become trained to do that. You become good at doing that. As a passenger, sometimes, the chimp starts to drive behaviour and you see that all the time with air rage or with people just becoming irrational about turbulence.
'I read a really good quote the other day from Mike Tyson, about a plan, fight plans. He said "everybody's got a plan, until they get punched in the face". And I think turbulence is a bit like that, in so much as a passenger might be able to rationalise it when they're on the ground and say "I know it's fine, I know that it's safe, I know that the pilots are great at it".
'But put them in an aeroplane in the middle of the night and start bouncing it up and down and suddenly the Mike Tyson punched in the face thing happens and they succumb to their irrational fear.
'Sometimes the calming voice of the pilot will help with that. And sometimes it just won't help at all.'
The pilot reassures passengers that they should not feel embarrassed about their flying fears.
He says: 'Anyone concerned with air turbulence should not feel embarrassed about it. I think maybe 10 or 20 per cent of passengers have a genuine fear of turbulence in a way that is not rational.
'Because in terms of what it might do to you, yes it's unpleasant, nobody likes being bounced up and down like that, or very few people do, but it's not unsafe.'
So what can turbulence do to the airframe?
He says: 'Nothing. Before the airframe gives way, your head will have come off. By the time airframe breaks up you'll be dead anyway.'
He reveals that pilots basically have four options for dealing with turbulence.
He says: 'You can grin and bear it - the aircraft is more than capable of withstanding the loads associated with turbulence - although severe turbulence can be quite uncomfortable and best avoided for passenger comfort. You can try flying higher - if aircraft performance allows it - or lower - although this burns more fuel and might make things worse.
'You can fly at the aircraft turbulence penetration speed - generally a little slower than normal cruising speed or turn to avoid the area of turbulence if it’s localised, such as near a thunderstorm.'
HOW DO PILOTS DEAL WITH TURBULENCE?
The serving airline captain reveals four methods for dealing with turbulence:
1. Grin and bear it - the aircraft is more than capable of withstanding the loads associated with turbulence (although severe turbulence can be quite uncomfortable and best avoided for passenger comfort).
2. Try flying higher (if aircraft performance allows it) or lower (although this burns more fuel and might make things worse).
3. Fly at the aircraft turbulence penetration speed - generally a little slower than normal cruising speed
4. Turn to avoid the area of turbulence if it’s localised (such as near a thunderstorm).
(culled from www.dailymail.co.uk)
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