14/05/2007

Un petit message.

Bonjour à tous.

 

Voici une nouvelles qui j'espère vous fera plaisir, je suis devenu Commandant de bord dans la socièté qui m'emploie.

Je peux vous assurer que c'est un travail très long, très accompli, et très demandant. C'est normal me direz vous. Je vous dirais de même.

Le texte qui suit est un briefing que j'ai donné à un de mes instructeurs. Il est en anglais, ne prend pas en compte la totalité du sujet, mais reste assez opérationnel pour un professionnel. Il s'adresse au départ pour un néophyte, et la conclusion s'adresse à un professionnel.  J'insiste sur le fait que ce texte ne regarde que moi et l'expérience que j'ai de l'aviation.  Cela s'applique principalement à un gros porteur, équipé de CF-6.Les critiques sont les bienvenues, que ce soit négatives comme positives.

Bonne lecture.

 

THE  DESCENT.

 

 

INTRODUCTION.

 

            What is following only engages me and my foreknowledge on the subject. Any constructive comments are welcome.

 

            There is an ideal descent that exist out in the sky during our operations, ideal because it does exists following the rules of aerodynamics and engineering, also ideal as it is really difficult to achieve due to ATC restrictions, so seldom reached.

 

            This ideal descent , with throttles all the way back to idle, ( known as flight idle), which is in relation to actual descent, speed reduction and proper aircraft configuration,  will get us to be  stabilized at 1000’ in IMC  or 500’ in VMC, all checklists completed.

 

            To be able to achieve this descent, some prerequisite must be met.

 

 

PREREQUISITE.

 

            1°) It is known that standard ICAO glide slopes are 3° glides.

 

            2°) We know that        -1 Nm= 1852 m

                                               -1 meter = 3, 28 ft

                                               -So 1 Nm = +- 6000.

 

            Trigonometry gives us Tgt 3°= H/D                 H and D being in the same units.

            When the angles are small we can also say that the Tgt of an angle can be taken as its sinus (there fore in radians)

                        As so that Tgt 3°= 0, 052 407 79

                                         Sin 3°= 0, 052 335 9

                                               3°= 0, 052 359 8

 

            This gives a value close to 0, 05, which is 5 %…( or 1/20…)

 

            Why such a fuss about this 3° or 5%? The majority of the glide are 3°’s, this 3 ° glide comes from previous calculations (Boeing 737-200).

            Take a glider, his « finesse » will take you close to 17… Add to this glider some engines at flight idle, which will give our previously described residual thrust, and our f »finesse » is improved to close to the 20. (20 units in distance flown for 1 unit in height)

             Our engines are the kinds of HBPR (high By Pass Ratio) that are close to 5-6.

 

            It is acknowledged that we do not descent at max « finesse », as the airspeed would be too low for our kind of operation, therefore increasing the airborne time, this is fuel efficient but a catastrophy commercially and ATC wise.

 

              3°) Flight idle gives us residual thrust. For our engine, at sea level, it gives us a thrust approximatly equal to 7000 Lbs.  It is demonstrated that with altitude, bleed air, nacelle anti-ice and local pressure and temperature, that residual thrust at flight idle will vary.  For the simplicity of the purpose of this explanation, we will take an aircraft, with a « constant bleed » taken out of each engine. This helps us assume that thrust during the descent will therefore only be influenced by altitude difference from the start of the descent till its end.

 

 

FACTS.

 

As a generality, most of us are computing their descent by multiplying their hight's to convert it into a descent.

 

            Let’s say for example that with our 20 finesse, we start a descent as from 27.000.

            This gives us 20* 27.000, giving 540.000 ft in distance to achieve during our descent. in other words 90 Nm.

 

            Let’s get some equations back from our books to explain all this.

 

            F = L/D = W/D so 1/F = D/W this for finesse in descent.

 

            T + W sin § = D

                        Sin § = D-T = D/W if thrust equals zero

                                   W

                                                      =1/F = 1:20 (if F = 20)

                                          =0, 05 = 5 % = Sin 3 ° = =- Tgt 3°…

 

            So we have tried to find a simple rule a R.O.T. (rule of thumb) to treat our problem of distance calculation vs. our start of descent height. If each time we had to go through so many computations for our descent, headaches wood not be too far away.  As our FMS is not a help and does not fly a perfect descent for us, this R.O.T is more than welcome for us, night freight dogs, out there in the dark.

 R.O.T. = H/ D = Tgt § = +- Sin § = +- 1/F  giving us -D = F * H where H is in ft so is the distance

      D = 20 * H/6000’ where h is infeet

But the distance in Nm

                                D = H / 300 (If F is 20)

 

                                  D = FL / 3 as FL = H / 100.

 

            That’s it! Our descent planning, as long as our finesse equals an approximate 20, or gives us a 3° glide slope or a 5 % descent, take the FL and divide it by 3.

 

At the beginning of this chapter, a more simple demonstration of the need to divide the FL was given.

            Generally, most of the pilots will multiply their FL instead of dividing it. The divide factor, being the rigid and perfect way of doing our ideal descent will give us more distance vs. the multiply factor.

            As an example take FL 270, and compare a multiplication by a division.

            270/ 10 * 3 = 81 Nm for the descent.

            270 / 3        = 90 Nm for the descent.

                                   Comparison made, if a planned descent for a three degrees glide slope is multiplied, you’ll be 9 miles too far from the runway (beyond that runway, not before…)

 

This simple division is however posing some problems when you are starting your descent from a FL which is not a multiple of 3, you come with some odd and difficult to find numbers for us pilots, known as beeing lazy.

 

            We could to counteract this take then the following R.O.T.

            As multiplying is more evident. (Take it as a multiplification of 1 / 3… so 0, 33333)

                        D = FL / 3 = FL * 0, 33 = FL * 3, 3 / 10 = FL / 10 * 3, 3 = (FL / 10 * 3) + 10%

 

            So that at FL 270, following the above computation, you come to roughly 90 Nm…

 

Unfortunately, most of the pilots are not computing with the last 10%, which leads to some confusion as a good TOD (Top Of Descent), if you start your descent at FL 400, the confusion can be dramatic if the computation is not followed during the descent and no actions are taken to correct those missing miles…

 

 

WEIGHT INFLUENCE.

 

            Taking in the previous chapter the formula to compute the angle of descent §,

We have Sin § = D-T = in some ways our VVI.

                              W

 

                               The fact that the weight has an influence on the descent is pretty much explained in this formula.           - Decrease the weight, and our Sin § will be greater, so will be our VVI ( there’s a direct relation between Sin § and a VVI ) The more the aircraft is starting the descent at a greater weight, the more it is getting closer to it’s own max « finesse », the lighter it is, the more VVI it will get.

 

 

THE MOMENTUM, OR ALSO CALLED INERTIA. HOW TO SLOW DOWN?

 

            It is said that E = MC² (even though some theoreticians found that some how this is not the truth anymore, I’ll use this to supplement the following), this gives us a small idea on momentum and its causes based on speed and weight. This formula is in direct relationship with a moment T in time. The amount of energy calculated at T is valid only for that moment and for certain conditions.

            As the final aim of the descent is to be set for an approach, with the A / C configured and at the correct speed, it is wise to take in account the Total DRAG given by our A / C. This drag varies with the design of our flying machine, and its purpose. Tell me your aim, sweet machine, and I’ll tell you your Drag…

            This speed reduction will be in accordance with the above formula at a time T and the related drag at that same moment T (vectors working against each other… As long as the A / C engines are at flight idle).

 

            For example, if you slow down your A / C at FL 200 from 300 kts to 250, the distance will be bigger than the same speed reduction if at 3000’… There’s no secret at all, just the air density is playing its role. Even if the engines are giving more thrust at 3000’ than at FL 200.

 

Drag = 1 / 2 CL ρ S V² where as ρ = air density, and is in direct relationship with height.

 

 

            I got a bit lost there, concerning the weight and a speed reduction…

            Consider loosing speed at two different weights.

Lift = 1 / 2 CL ρ. S V² whereas CL is in direct relationship with the AOA (Angle Of Attack)

Lift equals Weight following the basic formula. Other wise in simple terms our A / C would not hold in the air.

Therefore L = W = 1 / 2 CL ρ. S V².  One factor almost forgotten is our small V², which is squared.

            For a same speed, but different W, only one thing will change, (same alt and S), it’s the CL…So as to increase the W, at a same speed, the AOA has to increase. BUT, remember that the F max is also in direct relationship with the AOA, and that’s a constant AOA… So at a constant CL, V² has to increase for that F max, or any F taken for the descent, as long as it’s the same. This brings us to fly faster in the descent…Getting more energy with our V².

Take different weights, compute the same degrees of descent for both weights, as the AOA is the same for those descents, the speed has to vary…

 

Back to E =M C², the mass,  expressed in Kgs  and increased by a few tons, will increase our momentum at time T.In level deceleration as well as in descending deceleration. It’s as simple as that. The only counter effect is the Total drag that will slow down our A / C.

 

 

FUEL ECONOMY VERSUS BEST DEAL TIME / FUEL FOR COSTS.

 

            It’s quite 2 distinct descents, one is aiming for the best fuel at destination, the other one is aiming for the best relation of speed / fuel for the costs.

 

For the first one, the best option is to fly the max distance with our engines at Flight Idle. So as to reduce before TOD the speed so as to hit straight at that TOD the perfect speed for our F max. This increasing our fuel at destination .You might think that this very long and very slow descent might consume more fuel, I invite you to take the FCOM 8 and compute by yourself the result from an idle descent started very early. You’ll be amazed to see the differences; remember that when descending, your Flight Idle FF will also increase… But still, this is the more efficient way to be fuel efficient.

 

For the second, The aim is to start the descent at the latest point  with cruise speed, and during the descent increase the speed so as to increase drag, this giving a more important Sin§, this giving a more important VVI ( D-T). This will help give the best ratio between

                                                     W

Time / fuel consumption, winning time both ways as you stay higher longer, so your TAS is higher longer ( versus your calibrated speed   ), and your increased speed in the descent helps in that direction as well.

 

There’s a restriction that however needs to be taken into account, cabin rate of descent is also a factor… A rapid descent A /C wise might not be followed by the cabin controller.

 

 

 WIND

 

            An A / C moves in a mass of air, that itself moves over the ground. The direction of those two vectors gives a GS (Ground Speed), which is the real speed of our A / C in comparison of the ground. Those added vectors will influence our descent planning, and be in some ways responsible for many bad planned TOD.

 

            So the best wind ever in aviation, apart for take off and landing, is tailwind (TW)… But as it does not always happen, we also have to consider the no wind (NW) effect on our descent, and the headwind (HW).

 

            The NW effect is in the basic stuff.

 

            For TW and HW, a standard correction of +- 3Nm / 50 kts is a good value.

 

TW is good as it increases your ground speed, gets you faster to destination, makes you start your descent earlier, and so reduces as well the fuel consumption. As the TW makes you start your descent earlier, there’s no reason to find yourself in trouble if you have started your descent at the correct time. As wind normally decreases during your descent, you will find that some descents do require a bit of throttle forward to keep a good glide.

 

HW is the full contrary of the TW. But in this case if the descent is started at the correct TOD, and wind suddenly decreases dramatically, Your glide slope will be a total mess and therefore some pilot actions to recover the descent planning has to be done.

 

 

ALL OF THIS, BUT STILL SOME UNDERSTANDING

 

            In our descent, the flight idle is reached all the way. Air density decreasing with altitude, your Flight Idle will be lower at high altitude than at lower altitudes. Your TAS is also higher.

 

            At the TOD, with Flight Idle, and flying a constant Mach (M), you increase your CAS to keep this constant M, so you pitch down a bit more. This has the effect to give you sometimes an impressive VVI, till you hit the SOP “in descent” speed of 300 kts. When reaching that speed and selecting IAS on AFCS, the pitch decreases, as well as your VVI, and depending on your weight will stabilize to around -2500’ to -1800’. This VVI will further decrease when reaching lower levels as your Flight idle increase as well. During your speed reductions, still clean and getting closer to VFTO (F max) this VVI will further decrease. Getting configured will increase your drag curve as profile drag is increased.

 

            This means that your descent is not a straight line descent…It is represented so, but in reality looks more like curves with slight bents in it…Each time you configure or touch the bleed of the engines, you act on those curves. The descend in itself acts on it naturally by increasing the density.

 

 

 

FMS DISTANCE VERSUS FIELD SLANT RANGE.

 

            That’s one big mistake that most people do in their descent planning, is to be influenced by their slant range distance versus their actual distance to be really flown.

 

            A simple example would be to compute a TOD in a straight in, Let’s say it‘s 100 Nm from the Rny. If you move that runway around in the plane you’ll find that TOD will always decrease in the slant range from the field, but that our FMS distance will always stay the same.

            Plan  gates (a flight window) to be set for a certain speed at a certain point from the aircraft; in the 100 Nm example from above, a close study of the jeppesen will help you determine your slant range TOD. You can still choose the FMS option, and start your descent with it, or compute the slant range TOD with your study. If you come up with a ground distance of approximately 20 Nm past the field, your TOD from the field will be then 80 Nm.

 

 

 

 

 

 

COMPUTE COMPUTE COMPUTE.  AND CORRECT…

 

            It’s not because you computed a TOD that your A / C will fly accordingly… That would be too easy… Help yoursef by always computing your descent.  The same way that you computed your TOD. This will help you be on profile.

 

The “gates” story will be the one thing really helping you, have some gates ready for your computations. If on a 360 H° during your descent but landing on Rny 18, then prepare a gate with a certain speed and altitude abeam the field, with a 6Nm slant range. (In UK… Some other countries will take you 10 -15 Nm abeam)

 

As you are doing a descent and your continuous computations are leading you to believe that you have to correct your glide, you have two options.

            -Being too low = Well, if you don’t correct early, there’s no danger, but a bit more fuel consumption at lower levels. Depending on your profile, the time you want to give yourself, an early correction is not really needed.

 

            -Being too high = Well,  that’s the time to correct quite early enough, because the law of physics will keep you further away from the glide, and if you do not want your descent and approach to end in a G-A, early correction is the best way to put you back on profile.

 

In both cases (TW or HW), it’s a GOP (Good Operating Procedure) to try to find the reason why your descent planning has not worked in the initial computation. And maybe to correct directly the tendency by going on the “other side” of your glide. This means try to “over correct” a bit your glide, to give you a bit more room for correct manoeuvres. Once that reason is found and correctly assessed, redo a computation. Remember gates? They need to be

adapted as well.

 

 

THE CDA (CONTINUOUS DESCENT APPROACH) PROBLEM.

 

            More and more, we pilots will have to face the CDA problems, as noise restrictions will increase more and more in many airports.

 

            First, what is CDA ?

                        The CDA starts as when you are taken by radar control, or on a STAR, by the airfield control of intended landing. The CDA is interrupted when you are level flight for

 2, 5 Nm. A CDA is a mandatory noise abatement procedure and should be applied to its max extend. Even if you continue to descend at 100’ / min to 200’ / min, you are still in a CDA.

                        There are two main reasons to interrupt a CDA, the first one is an emergency, the second one is the ATC control restricting you in a hold. Any other reasons could be a good reason, but might be questioned by ATC.

 

            Flying a CDA .

                        First case.  You fly a straight in… Being on radar vectors or even better flying the STAR straight-in (due to the alt restrictions on the plates, the planning is normally easily met.)

                        Second case, in the GOT example, you’re cleared for the approach for the Rny 23, coming from the west with the STAR. The track miles are again easily known with your FMS and a good study of the jepp plates.

                        Third case, you’re under radar vectors.  At the first radio contact, the track miles must be given or asked for, and the number in sequence for the landing. Those two pieces of informations will be quite helpful for your CDA. This is the most difficult of all the CDA’s as you have to approximate constantly compute or guess your remaining track miles, even better ask for it if unsure. The horizontal situation awareness must be to its maximum… (Distance to the centre line, where is he leading me to intercept the LOC ?  When will we be established on the GS ?)

 

            Do not forget that on a CDA, deceleration has to be done descending, as well as configuring. A level deceleration is not permitted to prepare you’re A / C for landing, but reducing the pitch and decreasing the VVI well. Use of the speed brakes as well.

 

 

            Personally, the CDA ends when intercepting the glide, and I usually consider to meet the GS around 6-8 Nm.

 

            There are four ( or more …) ways of flying the CDA, the first one is to be exactly at all the required gates, so that you are exactly on the glide at all times. But deceleration will be a problem, so at one moment you will have to deliberately go below the glide to then pitch up a bit so as to let your speed decrease

 

            The other option  requires a 100% awake crew, with constant computations, and can lead to being too high if Murphy steps in. This CDA is flown slightly above the glide, usually 500’ to 1500’. Use the speed brakes to initially help you to configure, and then help from the slats and flaps themselves. This way of flying, if well followed, will guarantee you that you will be in order for noise abatements. However, it could lead you to be too high and smoking on the speed. Remember that once on the glide slope, your CDA ends, so you have to get your own gates to be established on the glide, with your configuration according to the miles to go. It’s a difficult exercise and the consequences could be high for the company and you if not well flown. This is why we are so badly paid. -J

 

            The third option is to play on the speed. Remember that VFTO is guaranteed 30° of bank and safe till 4000’ MSL, so above, as said my instructor, VFTO + 10 guarantees you till 23.000’. So you have room to play till VFTO + 10. Initially start your descent with 250 kts, then if slightly low, recover the glide with pitch, letting the speed decrease to the VFTO + 10, then get back on profile. It’s a gentle way of flying the CDA, and it has the advantage to keep you close to the slats speed.

 

            The fourth way, and my preferred one is to use all of the above at some different times during the approach, and not being “squared” as to use one more than the others. A bit of flexibility is used and appreciated for that descent. Switching from one to the other is intuitive most of the time, but still the gates and computations need constant attention.

 

 

            There are some guidelines to follow however for a CDA.

 

                        Don’t come smoking hot on the speed… Give yourself some room to manoeuvre in that longitudinal vector.

                        The speed brakes, the throttles, early configuration, are all means to correct your glide for the CDA, just think as being on a constant glide slope… Same corrections, early enough, and smooth ones.

                        Your go-around option must always be in the back of your mind, ready to be used.

                        360°’s can be used but are not appreciated by the control, especially in a busy area. It’s best to require some weaving, or eventually some S manoeuvres.

                        If you feel that the controller is taking you too far out, ask a turn a bit early. Remember, you are not working against the controller, but with him.

                        Do not be too late in your briefing prior the descent, as your mind will mainly be focused on the CDA, all checklist and briefings must be out of the way soon enough.

 

ATC

 

            ATC can be a burden when flying your descent, they can propose you to land on the straight-in rny while you planned an opposite landing.

They can increase or decrease your track miles like that. And getting some information soon enough on the track miles as well as your “number in sequence” will help you reduce some questions on what is going on.

 “Always be ready” is a maxim I do apply when flying under radar control. There are no fixed equations with ATC. A good situation awareness is the key to success when working a descent under radar control.

 

 

YOU

 

Seldom said in the descent planning, but you are the main hand in this. This means that your level of experience comes in the subject, as well as your awareness, your level of tiredness, the environment assessment etc…This is the tricky part of the descent, because you are this special limit, and only you can assess it. Others will assess it, but only when you come to make mistakes. It’s a GOP to tell anyway when is your level of attention at the start of the flight, or if at any moment you do feel tired. But that’s an other subject, many times treated in specialized books and known as CRM.

 

 

 

CONCLUSIONS

 

            As you saw, all those factors are influencing a great deal the descent.  There’s in my opinion no definite ideal descent, in the dictionary term.

 

            BUT, there’s an ideal descent for each purpose the pilot wants to reach, as there is a different flight regime for each purpose… A pilot might very well start a flight pretty much with the standard speeds as required by it’s company SOP’s, but if he’s told to hold at a point and he knows that in advance, he will reduce his speed quite early enough so as to save fuel. If he had known his exact time of EAT (Expected Approach Time) right from the beginning of the flight, he would have flown slower straight from the start, thus saving fuel.

 

            It’s exactly the same for a descent. Each TOD computed should be in accordance with the related aim.

 

            However, it is true that there is a descend that we meet most of the time, that could be

Called a standard descent. This descent will depend on its operator and his total experience, as well as his aircraft experience.

 

 

            For this example, FL 300 will be used.

 

                        I personally compute my descent the following way.

 

                        -Always assume it’s at MLW (at landing), (will be corrected later.)

                        -Always assume No Wind, (will be corrected later.)

                        -No speed brakes in the planning. (Except for correction)

                        -To slow down the A / C from 200 Kts, it takes you 700’ on the glide, at 2500’, with no speed brakes

 

-         ( 300 / 10 ) * 3 = 90 NM

 

-         Add 8 Nm for deceleration. (From 300 Kts to 230 Kts)

 

-         Take 3 Nm per 10 tons below MLW.

 

-         Add / Take 3 Nm per 50 kts TW / HW.This is where good preparation is important for Wx study.

 

-         Take 1, 5 Nm per 10 tons below MLW for deceleration.

 

This is giving a TOD, always recomputed during the descent. I’m sure you have noticed that I multiplied, and not divided… Well it’s because I like to be slightly above the perfect and ideal computation, so that I can play with the speed first, then the speed brakes if needed… This very personal technique comes with experience and awareness. If I do feel tired at the time, I’ll give myself some room and add those 10%...

 

To help me during the descent my weapons are first to increase the speed, so as to increase the Total drag, which will in turn help me descent faster in a smaller distance.

Use of the speed brakes till I recover the glide.

Configure… Getting your slats / flaps out will help you raising your drag compared to your airspeed… But for that you need to slow down to get to your speed. This is burning as well distance. A solid computation will help me on that, especially anticipation. Here is a word seldom named in this paper, but very useful.

Even in full configuration and badly taken, a way of sorting the problem is to get to your configuration max limiting speed, so that you will again increase your drag… A simple R.O.T is to multiply your distance to the threshold, fully configured at 179 kts, by 6 in A.G.L. 6 Nm * 6 = 3600’ so if in LGG AT 6 Nm from the threshold, your max alt is 4200’ MSL, there fore you’re still able to hack it… This will not be a stabilized approach so for that you need to take those 500’ in VMC or 1000’ in IMC… So at 6 Nm in VMC it’s 3700’ max and in IMC it’s 3200’…

As a result, TOD computation is a result of knowledge, airmanship, company procedures, as well as a proper planning of the place to go to in relation with ATC, own capability, Wx, and so many other things that are making this job apart from the others.

And remember that you have weapons to fight inattention as well as adversity.

 

                                                                                           

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