Question regarding approach techniques..

[John Hill]

The topic regarding the B777 incident at LHR has raised a question in my mind and that is: "Does the typical landing approach of a heavy aircraft depend upon engine power being always available?"

I am thinking of the comparison with a glider approach which (if I understand correctly) is done with altitude in reserve using lift spoilers to keep on the desired path.

Thank you.

[henry crun]

The short answer is yes.

With the exception of a handful of airfields which have steeper approaches for various reasons, nearly all commercial aircraft approach on a 3 degree glidepath.
That works out to about 300ft of altitude per mile.

With wheels and flaps down they cannot maintain that approach path without power.

[tom775257]

Yes.

The jet I fly (A320) requires about 50% power (N1) for final approach once configured fully.

Regards, Tom.

[John Hill]

Thanks for those responses, it is not how I expected but then I never really had occasion to think about it much. Meanwhile, on the B777 thread I note the sniping and snarling regarding what would happen if flaps were retracted in that situation.:rolleyes:

But then the next question springs up and that is are those lift spoiling devices that pop up when the wheels take the weight ever partially deployed on approach to provide extra lattitude in the event of wind shear, power failure etc?

[flypy]

John:

They're called lift-dumpers for a reason. No stabilised approach should require the use of spoilers/airbrakes. On some occasions, spoilers are used during a descent to expedite things a bit but certainly not during approach. Spoilers seriously upset the airflow over the wing and you would need a very high throttle setting to overact it just to keep straight and level or 3deg path. In high wind or gusty scenarios the best defence is to add some airspeed, so perhaps Vref +10 or 15. That way if the wind drops off or changes suddenly, you have a buffer.

The only A/C I know of that has it's spoilers/airbrakes open on approach would be the BAe 146/Avro RJ, which has spoilers which open out at the end of the fuselage - the rear engined Fokkers (28/70/100) have this design as well. Most of the time a 146 will use these in late stages of approach, but then it is meant to be a STOL aircraft.

[soem dood]

As a complete noob (Hi, first post here!), I was amazed and impressed at the information available on the other areas of the forum regarding the LHR 777 accident. Thanks for allowing a 'civilian' to peruse.

I had wanted to comment to someone about the following screen grab, but so many other new people seem to be running around the 'real' threads, mucking them up, that I was trying to find a corner where I would not be beat up for making a quick comment:

Wow.

These guys were either very good, or very lucky, and most probably both. My hat is off for being able to thread the aircraft between two very unsavory places (parking lot and road), to hit the grass, especially if they were indeed having to maneuver at the last second.

http://i31.tinypic.com/153uwll.jpg
[single frame grab used under fair use doctrine for purposes of commentary]
Looking at those wheel furrows, he hit it just where he needed to, with both wheels at same time, the gear collapsed, and they rode in on nacelles the rest of the way...

Did I say: Wow...?

[PPRuNe Towers]

Ummm,

I think you'll find those very marked furrows were actually made by the engines. Get hold of a better shot and you'll see what I mean.

Oh, and welcome aboard.
Rob

[soem dood]

Dang. My first post, and already caught making an obvious technical error. :O

Thanks!
:oh:

[Rory166]

Hi John

Your question seems highly relevant to me as a mere glider pilot. The professionals are fully gened up on the details. It seems to me your question reveals that the normal commercial practice is fundamentally not failsafe regarding total loss of power at low level. with the benefit of 20/20 hindsight if they had retracted the gear they would have been better off (assuming it was even possible). But of course they hoped to restore power asap. As you rightly point out landing a glider is very safe because the airbrakes give you a huge range of gliding angles so you can land wherever you like within reason. I seriously doubt that the whole method of landing heavy aircraft is going to be changed because of this one incident. The fact that no-one was killed makes this all the less likely. I can envisage an approach where the glide slope is increased dramatically as the gear is lowered. This would presumably bring all sorts of new problems like ILS would not work, engines at idle could not respond quick enough if a go-around is required and so on. Personally I feel much happier with no engine and airbrakes well open that I can put away instantly if I need to.

Rory

[Chperplt]

<<<with the benefit of 20/20 hindsight if they had retracted the gear they would have been better off (assuming it was even possible). >>>


Retracting the gear of a heavy transport jet creates a great deal of drag. This would likely have resulted in the aircraft touching down earlier than they did, which would not have been a good thing.

[TheChitterneFlyer]

John,

The L1011 TriStar deployed spoilers automatically during the approach; the system being called 'Direct Lift Control' (DLC), which enabled the aircraft to maintain a more or less constant 'alpha' (body angle). Push forward on the stick and the spoilers opened further, dumping the lift and increasing the ROD; conversely, pull back on the stick and the spoilers retracted, enhancing the lift and therefore reducing the ROD. The advantages were twofold, firstly, as mentioned above, it gave a constant 'alpha'. Secondly, the engines were now 'spooled up' in preparation for any subsequent Go-Around.

Big heavy aircaft will glide just like any other aircraft... as the space shuttle adequately demonstrates. But do take a look on 'YouTube' at a Shuttle approach and you'll see just how steep that glide angle is. I obviously haven't chosen a great example by discussing the shuttle techniques because the shuttle does indeed have to 'pitch-up' massively during the last couple of miles to reduce speed and arrest the 'frightening' rate of descent.

A B777, or any other big aircraft for that matter with no engine power (or limited throttle response), would have to plan a similar technique... but from 600 feet and two miles it's too late to achieve that sort of profile, because all of the energy has been lost within the normal 3-degree planned approach path... it's not a recoverable situation.

As for bringing the flaps in a notch or two... absolutely NOT; it would simply have brought the aircraft much nearer to the stall and would have actually dumped the aircraft onto the ground much shorter than where it ended up... probably in someones living room in Hatton Cross!

The boys did good.

[Rory166]

chperplt

Thanks for the comment. Just so I am clear; the act of retracting the gear creates a higher drag than the fully deployed gear. Is that what you are saying? Clearly the fully retracted gear has much lower drag, are you saying they would already have hit the ground before this could be achieved?

Rory

[Rory166]

TheChitterneFlyer

You clearly know about how things are done but what is bothering me is why. I had always fondly imagined that a passenger aircraft could glide into a landing if the engines failed. Now it seems that the standard approach profile makes this impossible. Again I say why? Is there no possible standard approach that would leave sufficient energy for a no power landing at all times during the approach? The kind of thing that might work would be a much steeper angle with spoilers extended, if the engines failed the spoilers would be reduced to enable the same angle to be continued.

You mentioned the kind of profile that would be required for a no power landing. I had assumed that pilots were trained in exactly how to land with no power, is this correct?

Rory

[411A]

The stabilized (power on, if you like) approach technique is used in jet transport aircraft for a very good reason...very large rates of descent can develop quickly with swept wing jets, otherwise...witness the 777 at Heathrow.

No (or too little) power, bad results.
Of course, a large jet could be flown at idle thrust during approach, starting from a much higher altitude, but then you would have those high rates of descent to contend with...not good in a jet.

[Rory166]

411A

Thanks for the comment. Please excuse my ignorance but why is a high rate of descent not good in a jet?

Rory

[TheChitterneFlyer]

Rory,

Just like everything else in aviation, it's all about compromise. Firstly, a good stabilised approach will have eveything dangling and sorted out, which means that the aircraft is nicely trimmed so that only minimal inputs are required to fly the aircraft accurately. Contrary to much maligned public thinking, total engine failure approaches aren't really considdered within any flight training environment until the crew have become much more experienced on the aircraft type. All-engine flame-outs are indeed practiced, but normally at high altitude so that the crew can practice emergency engine relights... should a relight not happen, on any engine, you're then obviously committed to that very rare emergency procedure of actually having to land the beast somewhere. I recall an Airbus running short of gas over the Atlantic (a fuel leak) and carrying out a successful approach into the Azores; also another dead stick landing into an airfield in Canada... both very well handled by experienced crew.

If we were to routinely carry out 'high energey' approaches as a matter of course, I feel sure that there would be many more incidents than there are today, because believe me, trying to keep a big heavy aircraft in-trim and under control during that final slow-down and getting the gear and flaps out requires considderable skill and concentration. High rates of descent at low altitude whilst trying to control that other aviation dimension - that of inertia - is an artform in itself and not recommended.

Just to clear up the 'landing gear' misconception that was previously mentioned. Raising the gear does indeed cause quite a large, initial, drag penalty, because the gear doors have to open prior to the gear coming up and it's those doors which cause that increase in drag. Once the gear is safely tucked away and the doors have closed then of course it's a much cleaner airframe. As an example, for most aircraft which are carrying out a windshear recovery procedure (particularly close to the ground), most airline procedures recommend that you do NOT raise the gear until you have achieved a satisfactory airspeed and climbing away from the ground/encounter. The very reason for this is because of the extra drag caused by the gear doors opening, which would negate what you're trying to acheive... gain airspeed!

Hope this helps.

TCF

[Rory166]

TheChitterneFlyer

Thank you for the very clear and informative answer.

As I see it now a total engine failure on approach is one of those situations for which there is no good solution. Hopefully it is an extremely rare event. If it happens you just pick the least worst place to crash land. It emphasizes how very lucky everyone on BA38 was that this did not happen even seconds earlier.

Rory

[TheChitterneFlyer]

You see it perfectly my friend.

Whilst we build airliners with huge ammounts of redundancy inbuilt we cannot envisage every contingency. 20/20 hindsight would be a huge step forwards and if that kind of human technology was available I feel sure that the BA crew would have stayed in bed that day!

Even after several days of media reporting they (the media) still cannot get the events correct... after all, all they want is a sensational story; at whatever the cost!

A few locals in my local pub approached me today to ask for any explanation that I might have... I just gave them the plain and simple facts, but told in such a way, that the layman would understand, and they now realise how it was, and not how the press have told it!

It would be nice to have an aviation correspondent within the media who knew what he/she was talking about; instead, all we get it someone who understands the intricacies of 'deep sea basket weaving' who has kindly stepped-in to give his/her professional opinion to the editor in chief!!!!!!!!! Disgracefull coverage.

TCF

[John Hill]

I am glad I asked this question as it resulted in a very informative topic, thanks to all involved.:ok:

[llanfairpg]

John the only thing missing from the above explanations is that an aircraft has an unique drag curve, its U shaped as opposed to straight line as to say your car.

Swept back wing aircraft normally approach on what is called the wrong side of the drag curve and are therefore speed unstable Eg any reduction in airspeed increases drag even more causing a further speed loss.(this is a product of this UNIQUE drag curve)

Any aircraft with wings can glide to land but to make a safe speed stable approach in a swept back wing aircraft you need the assistance of thrust.

Once the aircraft has intercepted the ILS glidepath it is on course for a touchdown point 300 metres from the threshold on what is normally a 3 degree glide path which is around 300 feet for every mile out from touch down. EG 2 miles at about 600 feet above aerodrome level. In the latter stages of the approach more drag producing flaps and undercarriage will be lowered which basically means more thrust will need to be be added to maintain that glidepath. At around the normal height to have everything out and dangling and the speed correct, which is just inside 2 miles the aircraft is very thrust dependant so if you take any thrust away at this point the aircraft cannot maintain that glide path and will land short. However you have got one other workable option of some free energy and that is the excess airspeed above the stalling speed. Aircraft approach at a safe margin above the stall speed and it it is this margin that allowed this aircraft to limp onto the airfield. The pilot used this margin to modify the glidepath to get to the airfield but there obviously wasnt enough spare airspeed to maintain the correct 3 degree glidepath.This very abnormally slow airspeed also contributed to the low ground roll out of the aircraft after touchdown. If the gear had been raised the aircraft energy would probably not have been as effectively removed as it was on touchdown.

[John Hill]

Thanks llanfairpg, very informative and I even believe I understand most of it! I assume also that having the engines actually providing thrust rather than at idle means that if more power is called for they will respond more quickly (thinking of big diameter turbines here)?