Gremline Flight Safety Report: Landing Errors. Cessnas, Aquila AT01, Socata TBM 700B

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the gremline digest — accidents on landing

Stop Crashing the Cessnas!!

Scanning through the Air Accidents Investigation Branch reports for the past few years shows a remarkable number of landing accidents involving single engine high wing Cessnas. Some land nose wheel first, others bounce and shear the nose leg off, some run off the edge or end of the runway and some even manage to end their ‘landing’ upside down. Why?


Let it be said that there is nothing wrong with the Cessna family of single engine high wing aircraft. I have only flown four models, the 172, TR182RG, 206 and 208. I had a Cessna TR182RG registered G-OAST for a couple of years in the early 1980s and enjoyed its altitude performance as well as its ability to handle turbulence and crosswind landings. My last flight in G-OAST was in April 1983 when I flew it on icy airways to Strasbourg in IMC for 3:45 hours and landed off an ILS at minimums. A very nice French gentleman was waiting with a bag of money and G-OAST was another cancelled UK registration. The Cessna 206 on floats is probably as much fun as can be had in pure flying over a rugged landscape. The Cessna 208 is the best utility aircraft ever built. So why do people keep on crashing their Cessnas?
Statistics are against the Cessna 150/172 family. The chances are that if some semi-skilled pilot is bumbling around the sky, then he or she will be in a Cessna. There are an awful lot of little Cessnas up there! The same problem faced the DC-3/C47 Dakota in the early 1950s. A large number of Dakotas were crashing around the world and some folk began to get their knickers in a knot and damn the DC-3 as unsafe. It wasn’t, and still isn’t. There were just an awful lot of them in the air.

      So, back to my earlier question, why are so many Cessnas coming to grief when they get close to the ground? I believe the answer is that many pilots simply do not know how to control their aircraft. They have either not been taught properly or have forgotten anything they were taught. We all forget parts of our training and develop bad habits with the passing of time. Only two kinds of pilot need regular revision training with an instructor. Those in regular flying practice and those not in regular flying practice. How long is it since you had a check with an instructor?

 

 

Drag vs Airspeed
I believe that many pilots either forget, or never understood, the need to fly an accurate and stable approach in order to get an accurate and safe landing. You must understand, and apply, the technique involved in flying your aircraft at speeds below the minimum drag speed where the aircraft is on the “wrong side of the drag curve” and is suffering from “speed instability.” Let’s refresh a few memories by looking at what is meant by the “wrong side of the drag curve” and “speed instability”. A little graph of drag versus airspeed is needed to illustrate these two vital concepts.

 

 



 

 

DRAG




 

 

min drag





                    

 

 

 

 

 

 

 

                                             endurance speed           AIRSPEED

Text Box: Figure: Drag vs Airspeed

This diagram illustrates profile drag with a green line, lift induced drag with a blue line and total drag with a red line. Profile drag (green line) increases by the square of the airspeed increase i.e. doubling the airspeed gives four times the profile drag. The lift induced drag (blue line) is not quite so simple to explain, but for the purposes of this discussion all that is necessary to understand is that lift induced drag is at a maximum at minimum airspeed and decreases with increasing airspeed. That is a simplification, but serves for this discussion. If we then add the values of the blue line to the values of the green line we get the total drag (red line).
      What use is all this stuff? Let’s look at what we can learn from this simple graph. The airspeed at which the blue and green lines cross gives the speed for minimum total drag – which is the speed at which we will require minimum power in level flight to maintain level flight. That, by definition, is our endurance speed that will produce minimum fuel consumption per hour. This is not the same as “range speed” which is the speed at which we consume minimum fuel per mile. Stick with it, we are coming to the interesting bits.
If we are flying along at a speed greater than our endurance speed (which is normal for cruising around) then we will be operating somewhere on the total drag curve (red line) where the orange arrow points to the total drag curve. If the aircraft now hits a little turbulence the airspeed will decrease slightly and our operating point moves slightly to the left. Notice that, with this decrease in airspeed, the total drag DECREASES. Thus we will already have excess power applied and the airspeed will automatically increase to return to the previously set speed. This is called “speed stability.” Next let us have a look at what happens if we are flying along at an airspeed LOWER than our endurance/min drag speed, as indicated by the black arrow pointing to the total drag curve. If the aircraft now hits a little disturbance and the airspeed decreases then the total drag will INCREASE. This will cause the airspeed to decay further and it will continue to decay right down to the stall until you either lower the nose or add more power or do a bit of both. This is called “speed instability” and it occurs whenever you are flying “on the wrong side of the drag curve.”

 

 

The Drag Curve — Takeoffs & Landings
When are you going to be flying on the wrong side of the drag curve?
On each and every take-off and approach to land. It is a basic rule of thumb that the approach to land is flown at 1.2Vs. That is 1.2 times you basic stalling speed. If your aircraft has a basic stalling speed of 50kts then you should fly the approach at 60kts. If your Vs is 60kts then approach at 72kts – fairly easy to remember!  When did you last check the stalling speed of your aircraft? Not what it says in the Manual but the actual speed at the actual weight of your actual aircraft.
      The important lesson to remember from my little drag v. airspeed graph is that you will always be “on the wrong side of the drag curve” and suffering from “speed instability” when taking off and when approaching to land. Remember your basic training that POWER controls your rate of descent on the approach and ELEVATOR controls your airspeed. The trap for inexperienced pilots is to get low and slow on the approach and instinctively raise the nose by applying up elevator. This will only reduce your airspeed even further and precipitate a sudden arrival in the undershoot or a ton of bricks thump onto the runway, with yet another bent nose-leg and mutterings of  “It must have been windshear.”
      Too much speed on the approach is just as useless as too little speed. If you come racing down the approach at 1.2Vs plus ten knots for the crosswind plus ten knots for the wife and kids don’t be surprised when the sorely abused aircraft strikes the runway a glancing blow with the nose wheel, folds the nose-leg backwards and darts off into the far undergrowth. If it is extremely turbulent on the approach it may be necessary to increase your airspeed by a few knots and to select your touchdown point a little further into the runway – or it may be wiser to go away and come back another day if you think the conditions are beyond your abilities.
      Finally let’s spend a few moments thinking about the actual touchdown on the runway. It’s not good practice to arrive a few feet above the ground, close the throttle and switch your brain off. When I was doing my basic flying training on Harvards my instructors drummed into my head “Remember, the landing is not complete until the wheels have stopped turning.” That was vital on Harvards as they could bite your bum at very slow speeds on the runway. It is a very useful thought for any pilot in any aircraft.
      Arrive at the selected touchdown point at the correct airspeed in the correct attitude after a stable approach. Continue to rotate the aircraft gently so that the main wheels contact the ground with a gentle kiss. Continue to fly the aircraft after touchdown. Lower the nose-wheel gently onto the ground and continue to control the aircraft as you apply gentle braking. Leave your after landing checks until AFTER the landing is complete. All the above may be applied to any aircraft type.
And for goodness’ sake STOP CRASHING THE CESSNAS!

 

 

Basic Mistake – Aquila AT01
The pilot planned to fly her Aquila AT01 from Heacorn to Lasham with a passenger on board. She got a weather update for Lasham from the resident tug pilot there. The surface wind was forecast to be 190/10kt with broken cloud at 2,500ft and 20km visibility.
Lasham is operated by the Lasham Gliding Society and their Airfield Manual has a section on VISITING LASHAM BY AIR that states:


”Runway 05/23 is the Medium Runway and runs north-east/south-west. It stands out well from the air but the surface is rough and is not used for take-off or landing. Visiting light aircraft will land on the grass centre triangle formed by the crossing of the hard runways … If landing on the south-westerly run or taking off to the north-east, turbulence van be expected due to the line of trees that you cross on landing/take-off.
      Visiting pilots should note that there is a great risk from undershooting in both directions and should therefore aim to land well up the airfield in this wind direction. If this runway is in use, the wind is likely to be strong and so there will be a wind gradient and turbulence.”


The visiting pilot commenced her final approach to Lasham at 500 ft and noted the weather conditions were as forecast although the windsock indicated little wind at the surface. During the latter part of the approach the pilot assessed that the aircraft was going to land short of the grass landing area and, with full flap selected, she raised the nose to extend the approach. The aircraft stalled at about 15 ft above the ground and landed nosewheel first. The nose gear collapsed and the propeller struck the ground. Neither occupant was injured. She concluded that the accident was due to insufficient speed on the final approach.
All the above information is taken from AAIB Report EW/G2007/09.22 which source is gratefully acknowledged.
      The following comments are not meant to represent the view of AAIB in any way and are those of Gremline alone. They are added with the aim of contributing to Flight Safety by encouraging other pilots to learn from mistakes already made instead of making mistakes themselves.
      The pilot’s candid assessment that the accident happened because she had insufficient speed on final approach is almost correct, but WHY did she have insufficient speed? Her own report gives the simple answer. She found herself going low on the approach
and raised the nose to extend the approach. She had forgotten her basic training. You control your approach slope with POWER, and you control you airspeed with ELEVATOR. If you raise the nose by applying up elevator without applying extra power the immediate effect is to reduce the airspeed – which is exactly what happened in this accident. During my basic training it was repeated over and over again “NEVER STRETCH A GLIDE.” This was in reference to a glide approach after an engine failure. Equally you cannot stretch your approach path by raising the nose (unless you have considerable excess speed already).
      Pilots are recommended to spend a few minutes reading and understanding the article
Slow Flying published in an earlier issue of Gremline. It explains in simple terms how to avoid the trap that caught the pilot involved in this accident.

 

 

Rushed Approach to Kidlington? Socata TBM 700B

A Socata TBM 700B on short final to land on Runway 01 at Oxford (Kidlington) Airport apparently rolled left through some 360 degrees from low level and crashed about 100 metres displaced from the runway threshold. The pilot and two passengers died on impact.


The Air Accidents Investigation Branch Bulletin 5/2005 covers the extremely detailed investigation into this accident under Reference EW/C2003/12/03. This report may be viewed in total at www.aaib.gov. The exhaustive investigation uncovered many relevant facts but the inspectors decided no definite conclusion could be reached as to why this crash happened. No technical evidence was found to explain the uncontrolled roll but there were certain operational possibilities. None could be fully supported without hard evidence, but loss of control resulting from an unknown distraction, or during the application of power for flight path adjustment or an attempted late go-around, were considered as possibilities.
      The facts in this report are based on the AAIB Report and that source is gratefully acknowledged. Any further conclusions or comments are those of the Technical Editor of Gremline and are not intended to reflect those of AAIB.

 

The Socata TBM 700B
The TBM 700B is a single engine aircraft with six or seven seats in a pressurised cabin. It is powered by one PT6A-64 free turbine engine producing 700 SHP and driving a four-bladed Hartzell constant speed propeller. The aircraft is certified for single pilot operation. It has a MTWA of 2984 kg and a maximum cruising speed of 300 KTAS at FL260. Large span coupled flaps reduce the stalling speed to 61 KCAS and have three positions; up, takeoff (10°) and land (34°). Roll control is by a combination of interconnected ailerons and spoilers operating through a cable and pulley system. The Socata TBM 700B also has a mechanical interconnection system that applies rudder when roll control is applied and also applies roll control (aileron and spoiler) when rudder is applied. This system allows coordinated turns to be commanded by the control wheel without the pilot needing to apply rudder. The engine air intake has an electrically operated ‘inertial separator’ to protect the engine from ice and debris ingestion. This ‘inertial separator’ has two movable vanes that, when switched on, rotate to cause the intake air to execute a sharp turn causing centrifugal force to discharge any heavier particles overboard. The inertial separator is normally activated as part of the ‘before landing’ checklist.

 

The Flight
The aim of the accident flight was to fly two passengers from Brussels International Airport to Oxford Airport and to return them to Brussels on the following day. The aircraft was registered in the USA and was not certified by the FAA for ‘Commercial-on-demand’ operations as this flight was subsequently categorised, although the commander recorded it as a ‘Private’ flight. The commander held a FAA Commercial Pilot’s Licence with a total experience of 1,573 hours of which about 500 hours were on type. He had not flown the TBM 700 for three months prior to the accident flight, although he had flown a light piston engine aircraft in Florida in the previous month.
      On the morning of the accident the pilot flew the TBM 700 from Liege Airport to Brussels with another pilot on board. The accident pilot operated the aircraft throughout this flight and landed at Brussels at 0840 hrs after an autopilot ILS approach to 250 feet agl in a 300-foot cloud base. The autopilot was disconnected at 250 feet agl and a manual landing was made at about 85 kt using full flap. The aircraft was then refuelled to full and the second pilot left before the two passengers arrived.
      The aircraft left Brussels on an instrument departure at 1017 hrs and climbed to FL240 without any unusual event being noted by ATC. A descent to FL120 was begun at 1052 hrs and this level was maintained until a further descent was begun at 1110 hrs. The aircraft had reached about 2000 feet amsl by 1120 hrs and was requesting radar vectors towards Oxford from Brize Norton Radar. The Brize controller turned the TBM 700 from its westerly heading through about 270° left onto a northerly heading and cleared the aircraft to 2000 feet on 1029 mb. The turn was for separation reasons and left the TBM 700 heading directly towards the active Runway 01 at Oxford at 4 miles range. The pilot reported visual contact and was transferred to Oxford Tower. The pilot reported three miles visual and the controller cleared the aircraft to land with the surface wind of 030/15 kt. Nothing further was heard from the accident aircraft.

 

 

The Accident
Several experienced pilots were among the witnesses who watched the aircraft during its approach. One witness said that the aircraft had descended to about 50 feet agl when it began to roll to the left, with the nose rising as the bank angle reached 60°. The roll continued and the nose dropped with the roll continuing through almost 360° before he lost sight of the aircraft as it struck the ground. Another witness had his attention drawn to the approaching aircraft by a considerable increase in engine noise at it passed directly overhead. It was rolling to the left with about 40° when he saw it. The roll continued quickly to beyond 90° left before the roll reversed to about 60° left. The aircraft turned to the left before losing height and striking the ground in a nose low, left bank attitude. Most of the witnesses thought the aircraft was beginning a go-around just as, or just after, the aircraft started to roll to the left.
      The sequence of events during the approach is reconstructed from the evidence of a selection of witnesses, on the ground and in the air. Runway 01 at Oxford is 270 feet amsl with a Landing Distance Available of 1200 metres and is 23 metres wide with an asphalt surface. The PAPIs are set to 3.5° and are located left of the runway 128 metres from the threshold. Heathrow Radar provided a final contact at 1122:29 hrs at a height between 1420 and 1520 feet agl and 2.6 nm from the threshold. The aircraft struck the ground adjacent to the threshold at 1124:40 hrs giving an average groundspeed of 93 kt and an average rate of descent of 870 feet per minute throughout the approach.
      The aircraft was about 131 kg OVER the Maximum Takeoff Weight of 2984 kg with the CG at 32.2% when it took off from Brussels. At the time of the accident the estimated weight of this TBM 700 was 2942 kg (just 42 kg below MTOW) and the CG was at 32.5%, within the Flight Manual limits of 19.5% to 36%.
      The Pilot’s Operating Handbook recommends that a minimum of 10% torque be maintained until the landing is assured. This is to ensure positive and rapid engine response to throttle movement. Normal approach is at full flap and 80 KIAS. The stall speed at the accident weight and configuration at idle power would be at 61 KIAS.

 

A Tentative Analysis
It is possible to reconstruct the approach path and profile of the accident aircraft by reference to the combined evidence of the pilot’s last radio transmission, radar recordings, of air traffic controllers and of numerous witnesses on the ground close to the accident position. The aircraft should have been flown down a 3.5° glideslope at 80 KIAS with power set not below 10% torque. The surface wind during the approach was 030/15-25 kt and the average recorded groundspeed of the TBM 700 for the whole approach was 93 kt, giving an average indicated airspeed of about 108 kt, twenty-eight knots above the recommended approach speed. The aircraft was always some distance above the ideal approach slope in that it was at 1730 feet above airfield level at four miles range (about 5.5° approach slope), at 1470 feet QFE at 2.5 miles (about 6.4° approach slope) and at 540 feet QFE at 1mile range (about 5.8° approach slope).
      The significance of the combination of excessive airspeed and the steep approach slope added to the heavy weight and aft CG of the aircraft becomes clearer when we try to visualise the situation the pilot found himself faced with in the late stages of the approach. The aircraft was fast, high and heavy. It is likely that the pilot had the power lever fully retarded. Oxford Airport had a NDB and a DME positioned close to the threshold of Runway 01 but did not have an ILS for the pilot of the TBM 700 to use for an autopilot approach. This was apparently his first ‘manual’ approach in the TBM 700 for at least three months, having used the autopilot to fly the earlier approach into Brussels International Airport. It appears that the whole landing procedure may have been rushed and the aircraft never achieved a ‘stabilised approach’ condition. The pilot may have made a late decision to go around by beginning to apply up elevator at the same time as he advanced the throttle. It is possible that the engine response was slow because the inertial separator was operating (as is normal during the approach) and because the pilot had selected less than the recommended minimum of 10% torque in an attempt to reduce IAS and to lower the glideslope angle. Post crash examination found the pitch trim consistent with an airspeed between 111 kt and 114 kt. The aircraft would have been out of trim at the normal approach speed of 80 KIAS and the control wheel would have required a pull force of between 11 and 13 pounds to counter the nose-down trim. Any abnormal flight condition could have been exacerbated by this out-of-trim condition.
      There was no obvious technical reason for a left roll at the final stage of the approach so the AAIB investigation reviewed other possible reasons for the loss of control and looked at evidence for and against these possibilities. These included some sort of pilot incapacitation, a distraction, fuel imbalance, icing, wing stall and loss of control during a go-around. Despite an extensive investigation, no definite conclusion was reached by AAIB as to why this aircraft crashed during a visual approach to Oxford (Kidlington) Airport. There were certain operational possibilities but none could be fully supported.
      The aircraft would have stalled in normal landing configuration, with idle power and wings level, at about 61KIAS. This would have resulted in a left roll/wing drop as observed, but would require the airspeed to be some 19 kt below the normal approach speed. The recorded radar information indicates an average airspeed of about 107 kt during the approach, some 27KIAS ABOVE normal, but these radar recordings did not extend to the latter part of the approach, so it is likely that the pilot found it necessary to make a considerable reduction in airspeed while still attempting to reduce the angle of glideslope towards the end of the approach. These two requirements are in opposition, thus leaving the pilot with a very difficult control problem. There is also a possibility that the pilot suffered some distraction at a late stage on the approach.


It seems possible that the pilot failed to control the aircraft because of the unstable nature of the approach, resulting in a departure at an altitude so low that recovery was impossible. This conclusion is that of the Gremline editor and not that of AAIB. It is the opinion of the Gremline editor that specific type training is required for the safe operation of such high performance aircraft.

 

 

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