Nose Wheel Shimmy & Decision Making


Ignorance is the curse of God; knowledge is the wing wherewith we fly to heaven.
William Shakespeare

Aircraft performance is always better calculated before flight than during it!
Aircraft performance is always better calculated before flight than during it!

This article is based on a very interesting accident report describing an incident that took place at Montserrat involving a Britten-Norman BN2A-26, Islander, VP-MON. The incident was caused by nose wheel shimmy which eventually resulted in the pilot steering the aircraft off the runway onto the grass to avoid a possible runway excursion and overrun down a hillside.

The full report is here:

http://aq.be/31b415

The first time you experience nosewheel shimmy it can be quite frightening especially if you are not sure what it is. The nosewheel oscillates from side to side quite violently causing large vibrations through the front cockpit area although it feels much worse than it actually is. Nosewheel shimmy normally only occurs during the landing roll. The problem is that it can be very distractive and DISTRACTION is a major cause of incorrect aviation decision making.

As pilots we need to try to be able to deal with making rational decisions while being distracted but it’s definitely not as easy as it sounds. For instance, learning to deal with smoke in the cockpit is a doddle in your living room but put yourself in an aircraft at several thousand feet above the ground, with your eyes watering so you cannot see properly, with the passengers screaming and ATC asking what your intentions are will distract you to a point where even simple decision making can go horribly wrong, believe me!

In this incident the inspectors report shows that the pilot used two incorrect techniques.

1. GPS ground speed on the approach (Use the ASI for speed control on the approach) Accurate threshold speed is critical on performance limiting runways

2. Holding the nose wheel off on landing, and delaying braking and then failing to apply continuous braking. It’s essential to get the nose wheel down straight away and apply maximum effort braking on a performance limiting runway. I also suspect this pilot could have touched down earlier than he did, accurate early touchdown is vital on short runways but never compromise any obstructions in the approach glide path funnel by ‘ducking under’. If touchdown accuracy looks unachievable always immediately initiate a go around on a performance limiting runway.

As the report also importantly mentions, once you stop continuous braking you cannot be sure how the landing roll out will be affected. You cannot accurately measure landing roll out distance on ‘stop start’ braking, you can only measure roll out performance on CONTINUOUS braking that doesn’t lock the wheels, in other words, maximum effort braking. This is something to consider when undertaking a performance landing anywhere. Get the aircraft on the ground as early as safely possible and get the brakes applied as soon as possible using continuous pressure until taxy speed is reached, only briefly release the brakes if you experience a wheel locking up and after release immediately reapply the brakes again. You must apply CONTINUOUS braking pressure on a runway where distance is limiting if you wish to achieve the calculated performance  figures.

The AAIB report is not comprehensive in its recommendations, in my opinion, all though it’s good to see they have mentioned the possibility of flying a go around which may always be a solution to any landing problem although most pilots would probably not consider this after touchdown. You must however ensure THAT YOU HAVE SUFFICIENT DISTANCE REMAINING TO BECOME SAFELY AIRBOURNE. This is where judgement is critical, you have to eyeball the situation for yourself, you need to be able to get back into the air within the remaining runway and you also have to consider OBSTACLES in the climb out path especially if you are flying from a private strip as trees and hedges can sometimes be found in the most inconvenient of places!

In a situation such as this incident the decision making process has to be completed in a split second. Good decisions are usually only made from a sound knowledge base coupled with good situation awareness and adequate pre flight planning (in this case local runway/airfield knowledge, performance and weather). Rapid decisions such as this do not have time for review (DODAR – Diagnose, Options, Decide, Act, Review.), you generally have to get it right first time. If you are a student or low houred pilot it’s unlikely that you will have previous knowledge or experience of the situation you find yourself in. This is why good comprehensive flying training with an emphasis on Threat Error Management (TEM) and regular revison are vitally important.

For instance, how many of you reading this article had nosewheel shimmy properly explained to you before first solo or even know what it is now? If you have been correctly taught the pre flight inspection you should already know about the anti shimmy damper on the nosewheel oleo leg and its purpose. It’s wear in this damper that can lead to nosewheel shimmy although this cannot be detected by visual inspection of the damper.

Knowledge of what lies beyond the runway and to some extent either side of the runway is often overlooked in single engine operations. It’s always better to find out at the pre flight planning stage that there is sheer drop into the sea after the runway rather than as you sail over the cliff on an abandoned take off or an attempted late runway go around!

Abandoning a take or initiating a runway go around needs some knowledge of the runway distance remaining and how many pilots actually know the last point on the runway at which they could safely get back into the air or stop if they had to? It’s something as instructors we have to think about frequently as some students may land ‘deep’ during touch and go’s and especially at night when the situation is far more difficult to judge. Knowing the approximate mid point on a runway you are using assists this decision making process as does working out the stop distance in your aircraft and having a rough idea where that position is on the runway you are using. So many pilots never give a thought to what can go wrong that when something does go wrong they are completely overtaken by distraction and surprise. Aviation Decision Making (ADM)is a unique knowledge based skill that needs regular practice. Some people are naturally good at making decisions, others are not but which ever category you fall into you need to plan for the worst and hope for the best!

In this situation I would have most likely have braked continuously through the shimmy to a high taxy speed together with using full up elevator to take the weight off the nosewheel Using the elevator to aerodynamically remove the weight from the nosewheel isn’t mentioned in the report which is very surprising as its such an obvious thing to do. It is the weight going onto the nosewheel that exacerbates nosewheel shimmy (this is why it normally only happens on landing), so aerodynamically removing some of that weight helps prevent the shimmy or at least lessens it.

On reaching a fast taxy speed (forget Tiger Moth instructor folk lore of taxying at fast walking speed, taxy speed is dependent on and proportional to circumstance and situation) I would then have decided if enough runway remained to stop and then would have considered steering off the hard surface as a last alternative. Why? The aircraft was well capable of landing on this runway using the public transport factored distance, the increase in roll out distance was caused by holding the nosewheel off too long and releasing the brakes unnecessarily. Therefore straight line braking was well possible and desirable especially if steering off the runway looked inevitable as in my opinion it’s better to steer off at the lowest speed not the highest!.

Always plan a performance landing (short field) as just that because if something does happen, as in this incident, half way through the declarative roll out, you have more time and space to deal with it. This pilot put himself into a precarious position by wrong technique and poor technical knowledge, in my opinion. Yes, he did achieve a safe conclusion but any time you leave a runway by an abnormal route you risk severe airframe damage which in turn can lead to fire, as many off runway excursions have proved in aircraft of all sizes. The best place to stop an aircraft is in a straight line on a runway, that’s what runways are constructed for! However if you feel there is insufficient runway remaining and there is a big surprise waiting for you at the end of the runway such as a cliff or hillside, of course you must consider a deviation to keep the aircraft intact but this must be a last alternative. The emphasis is on ‘intact’ here and of course you do know what obstructions are on each side of your home airfield runways if you ever had to take this action? Umh, I wonder!

Shimmy is speed related and will only happen within a narrow speed range, releasing the brakes will most likely prolong the shimmy as it will keep the aircraft within the critical speed range for longer. Maximum effort braking will very quickly take the aircraft through the critical speed and stop the shimmy, something the AAIB inspector has also failed to mention in his report! I’ve experienced shimmy on aircraft and on both racing cycles and motorcycles. On motorcycles high speed shimmy is very frightening and can be very dangerous but again it is speed critical and once the machine is accelerated or decelerated below the critical speed the shimmy will stop.

Threat & Error Management (TEM) is part of ALL EASA flying training syllabus’s including the PPL and one of the best ways of making a start on TEM is to learn by the mistakes of others. Reading accident bulletins is a good way of learning about TEM and something that all pilots should consider as part of the ongoing learning process, so why not sign up for the AAIB monthly updates. Knowledge is the only cure for ignorance!
http://www.aaib.gov.uk/

February bulletin
http://www.aaib.gov.uk/publications/bulletins/february_2014.cfm

lion airAlways remember that the area past the end of the runway may not be suitable for aircraft deceleration!

Fly safe, safety is no accident!
All comments in these pages are my own views and are not presented as fact, merely opinion. You should ensure you are thoroughly up to date with the latest recommendations of the aircraft manufacturers and the CAA.

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