Gunnar Fahlgren

 

The airflow – the wind – around the aircraft produces the lift which is necessary to make our plane stay in the air.We actually like the wind. A strong head wind, during our take off run improve ou performance. 

How can wind cause problems?  

The answer is PHYSIOLOGY and PSYCHOLOGY. 

The effect of the wind is physiology and the cognitive reactions in the pilot’s brain are psychology.  

 

The tail wind approach and landing.  

Now I will describe a "wind accident" which now and then can be read about in newspapers and accident reports. /The latest was a Boeing 737  landing in tailwind February 05, 2020 in Istanbul.

This type of accident might happen to both professional pilots and less experienced pilots.  

Most of our approaches are made in head wind. The wind component is then normally a bit stronger at initial approach altitude, 1500/2500 feet, than on the runway. 

A great majority of all approaches are made under such conditions. And the pilot, or his auto throttle, will gradually reduce power, hardly noticeable, when he is approaching ground and his head wind is reduced.  

But what will happen in a tail wind approach?  

Most airport procedures accept landings with a tail wind up to 10 knots. 

That authorization gives a false impression that a tail wind approach won't cause any problems.

But let us analyze the approach step by step.  

Let us say that you are going to make a landing on runway 01. The surface wind is 180 degrees at 3 knots. 

A tail wind component of 3 knots should not cose a problem – or?  

But at 2000 feet above ground the wind might be 180 degrees, 24 knots. 

The main change of wind speed, due to the orography, will usually occur at around 400 feet elevation. 

If the aircraft is stabilized on glide path in a fully automatic approach the following will take place.  

 

1. Initially you will notice a much higher rate of descent than normal

2. When stabilized on speed, let us agree upon 140 knots, with full flaps and gear down, you have throttled back to a much lower power setting than normal. (Your 24-knots tail wind is acting as an extra engine pushing you forward).  

3.  When you approach 400 feet and the tail wind rapidly decreases, your airspeed indicator will – well what do you think? Yes initially it will indicate a higher speed let us say 152 knots.  

4. The response to this increased airspeed, from your skilled brain will be a power reduction to get back the desired speed of 140 knots.  

5.  A few seconds later, with your aircraft still on glide path, your airspeed is rapidly dropping. It is falling not only to 140 knots but probably to 130 knots. The angle of attack is increasing. The drag is increasing enormously.  

6.  Now you have to increase power fast and distinctly and you might even end up with take off power to recover your desired speed of 140 knots and reach the runway.

7.  After landing you will most probably say: 

– "Pugh, that was a hell of a wind shear."  

 Very few pilots will after landing scrutinize what actually happened. 

The fact is that you acted completely wrongly when you reduced power when speed increased. 

As your pushing wind – your "extra engine" – stops, you should INCREASE power instead in order to compensate for that power loss. Otherwise you will not reach the runway.  

 Pilots have been taught to reduce power, when speed goes above a desired value. That has been trained and trained for years. It has been stored in your brain, in your Motor Memory, which will act automatically. 

This wrong action has caused one or two heavy accidents every year.

Most accidents when a fully operative aircraft and crew hit the ground before, but rather close to, the runway or overrun the runway, I would say, are caused by this tail wind effect.  

Of course the tail wind is not the only reason for these accidents, but it is definitely a serious contributing factor. 

As a countermeasure the new generation auto throttles have now been modified to increase power instead of making power reduction. They get information not only from air speed but also from ground speed. 

As ground speed will decrease when tail wind is reduced, this auto throttle will compensate with more power and disregard the increase in air speed. 

This is exactly what human pilots should be trained for namely to increase power when speed is increasing! 

Air speed makes your aircraft fly.

Ground speed brings your aircraft to the runway.

 Things to be done to avoid this type of accident.  

1.  Avoid tail wind landings. Request another runway.  

2.  If you have to make a tail wind approach do not accept an initial approach altitude below 2500 feet. 

A longer approach will give you more time for preparation.  

3.  Be prepared to overpower your auto throttle.  

4.  Train your brain to respond with more power, when speed is increasing.   

5.  If you have to land in tail wind, ask the controller for actual wind at 2500 feet. Then you are prepared. 

 A good example, of what the effect of such a request might cause, is my approach to Copenhagen airport, Denmark. 

I was cleared to land on runway 04 left. 

Actual wind was tail wind 6 knots. The following conversation took place.        

Tower from SK 401: 

– Request actual wind at 2500 feet.          

– Stand by.   

– Standing by.  

(Ten seconds later)       

– SK 401, we are now changing to runway 22 left. Turn left to heading 020. Climb to 2500 feet and contact approach control.   

 

The problem is that tail wind landings will be more and more frequent. Because it is a great problem for an airport controller to change to another runway with a lot of traffic in his control area. 

If the wind is changing, the landings most probably will continue, as long as the limit of 10 knots tail wind is not exceeded, before a change will become actual.  

At the time of the SAS DC-10 accident at Kennedy airport, I have been told that they deliberately used a tail wind runway, because they had used the opposite runway the day before.

And the noise had to be evenly distributed due to a noise abatement request. 

This I regard as a typical unprofessional decision. And we must never forget that noise abatement procedures are, and have always been, in conflict with flight safety. 

Pilots who did not fly before 1959, when the first noise abatement procedures were implemented on the Caravelle, have never noticed the difference.   

 

Another problem with the tail wind approach. 

As said before, most of our approaches are made in head wind. 

A headwind, which is gradually reduced as we are approaching the ground. 

All those approaches will create unconscious "timing" which makes it possible to make safe approaches several times a day in extremely bad weather conditions and without any undue stress. This unconscious or instinctive "timing" is very important. I name it Internal Timing. This internal timing is highly activated for a short period during the final stage in the approach. We automatically start this internal timing and also internal Communication synchronized to that timing. We make several call outs at certain intervals. 

At specific intervals we select different flap settings, select and check gear down. 

We check decision altitude and altitude over the outer marker. We evaluate braking action in relation to cross wind. 

We check speed and sink rate at certain intervals. 

We, so to speak, live in harmony with actual conditions and are therefore capable of making sound decisions even under, what might be regarded as, adverse weather conditions and a high degree of stress.

 Our BTE-computer (Between The Ears computer) delivers good decisions based on Rules, Knowledge and Skill.

During well-trained and correctly expected conditions those three systems work perfectly together.  

 Tail wind  Psychology:

 Now let us look at this in a tail wind approach.   

Pilots start their internal timing and communication in the usual way. Everything seems normal. But gradually the crew will get a feeling that something is not correct. What causes that feeling?  

 Well, a professional pilot, like a professional musician, has rhythm in his body produced by a lot of training and experience. This rhythm is produced by internal timing, which controls his Motor Memory. 

The wrong rhythm will not give an orchestra any applause and distorted rhythm in cockpit work will seriously effect the pilot’s professionalism. So, the reason for their concern in a tail wind approach is that their internal timing and communication do not correspond to actual demand as, at this specific approach, 25 - 30 percent of the time, normally required, is just lost without their cognizance. 

This might cause cognitive disturbance or even a cognitive breakdown; depending on what degree the stress level has developed.  

A typical cognitive breakdown can be noticed in the tailwind approach at Katmandu airport, Nepal in July 1992, Airbus 310. 

This unnecessary stress is pushing the pilot higher and higher up on his stress curve. The wrong action to further reduce the reduced power, when speed increases to 152 knots, might, due to stress, result in a reduction in his mental performance.

 

First tunnel vision and then blocked perception of warning signals or call outs and finally reduced or blocked perception of all his five senses might occur. 

It is important to remember that if the pilot's stress and arousal level increase too much, his ability to make decisions also rapidly deteriorates.  Not only his ability to make good decisions, like ”going around” or handling a malfunction, deteriorates but also his situational awareness is going to be more or less blocked. It might even develop into a conflict between his own BTE-computer and his FMS (Flight Management System) 

A breakdown in the flight crew/automation interface has been noticed in many aircraft accidents.  

As said before, our perception of signals from our five senses is gradually reduced. Call outs and even a PULL UP signal from the GPWS (Ground Proximity Warning System) might not be heard.  In highly stressing situations when demands increase, our defense mechanisms, in rare cases, might pop up and DENIAL of a catastrophic situation might be a fact.  

 

In July 1992 an A310 approached Katmandu airport, Nepal.  The standard approach profile is steeper than normal, to this can be added that they had a considerable tail wind. So, physically, the A310 had a lot of pushing energy.

A technical problem occurs and the crew aborts the approach. That was a good decision, but the flight path thereafter indicates a mental breakdown. A few minutes later the voice recorder, which recorded communication between the Captain and his First Officer, clearly indicates an actual flight-crew-automation interface problem in the cockpit and soon after that a denial of facts. The GPWS starts to sound 17 seconds before the aircraft hits the mountain. This warning starts no action.

Another well-known accident is the ”Cali Crash” on Dec. 20, 1995, when a Boeing 757 approached Cali, Colombia. 

To save time the captain decided to change from a head wind landing RW 01 to a tail wind landing RW 19 . The stress increased to such a level that decision-making was impaired and situational awareness was lost.

Another effect.

In a tail wind approach one should also keep in mind that the preceding aircraft might very well still be on the runway when you are due for landing. 

A go around situation might occur much more easily in a tail wind approach than in a head wind approach, which of course increases the level of stress.  

 In tail wind approaches – well in all approaches – it is a question of: Physiology– Power, drag, lift, wind and the steepness of the glide path. And Psychology: Stress, defense mechanisms and cognitive dissonance.  

One disadvantage with a crash in tailwind is, that the fire will spread forwards and reach your cockpit as was the case in the tailwind approach with an Airbus 320 at Bangalore; India Febr.14, 1999.