View Full Version : Tech help reqd please.

Grace Quirrel
14th Apr 2002, 12:54
Hi folks.

I would really appreciate quick answers to the following questions if you don’t mind. These are possible interview type questions and I cannot find a definite right answer.

1) Can an increase in clearway increase MTOW and will an increase in clearway affect V1 by increase/ decrease/remain the same.

2) What effect will a change of Flap / Slat position (whilst maintaining a constant attitude) have on Angle of Attack.

3) Any simple calculations for Density Altitude given Palt and Temp.

Many thanks in advance.

The Greaser
14th Apr 2002, 15:41
Not too sure about 1 but I'll try 2 and 3 :

When you deploy flaps and slats then the aerofoil coefficient of lift increases. Thus to maintain the same amount of total lift given the same airspeed then you will need a lower angle of attack.

If I remember correctly, to get a rough estimate of density altitude work out the temperature deviation from ISA and multiply this by 120 and then add/subtract this to pressure altitude.

eg. PA 4000'
Temp +27 (=ISA +20)
DA = 6400 (4000+(20 x 120))

14th Apr 2002, 16:04
Don't forget the pressure altitude difference:


For example:

QNH= 1031

=> QNH-ISA=Pressure difference

=> 1031-1013=18hPa

=> 18 X 30 (ft X hPa) = 540 ft

therefore at ground level on this kind of day the aircraft will perform like it's 540ft underground ;) => increased performance.
Adding this to Greaser's case above (assuming QNH=1031 hPa)means that the density altitude corrected for both temp & pressure will be 5860ft instead of 6400ft.


Capt Pit Bull
14th Apr 2002, 16:29
Trouble with these kinds of questions is you have to be very specific about what is being asked. I'm not much a performance guru, so I'll tackle number 2.

Stated in Question - Constant Attitude.

Consider how Angle of Attack is defined. I don't recall the text book definition, but something close to the following:

The angle between the Relative Air Flow arriving at an aerofoil and the mean chord line of that aerofoil.

So, in the question, has Relative Air flow changed? I would say No, since the attitude of the aircraft is constant.

Has the mean chord line changed? Well, lowering a trailing edge device has the effect of tipping the mean chord line backwards, thus, by definition, increasing the angle of attack.

Lowering a leading edge device has the reverse effect.

That answers the question.

What happens next is that this change in AOA, plus the effect that changing the camber of the wing will change the lift coefficient, combine to change the lift generated by the wing, so the pilot will have to change the attitude or accept a change in flight path. This is what Greaser is talking about, and whilst he is quite correct he is not answering the question.


14th Apr 2002, 16:31
1. As I recall UK Perf A, a runways clearway can only be extended up to a certain point. It can be worked-out on a graph thats provided when making these calculations. A simple way to remember this requirement is to think of a TO from R24/25 at LAX. The clearway over the Pacific is 6,800 nm!

2. Angle between airfoil chord-line and relative airflow = AoA.

Maintaining the same attitude (body-angle) while deploying hi-lift devices:
Flaps will increase the AoA because it makes the chord-line (line from edge of flap to leading-edge) longer (lift-coeff) and at a greater "apparant" angle to the relative airflow.

Reverse is true when retracting these devices.

3. (Isa Dev x 120) + QNE = DH

Ex: QNH 1009 at airfield 5,500 ft. OAT -1 deg C:

a) 5500 at 2deg/1000 = -11deg decrease.
15deg at SL - 11deg = +4deg ISA at 5,500ft
+4deg - -1 = -5deg deviation (colder than ISA).

b) winding altimeter up 1009 to 1013 = 4mb x 30 ft = increase by 120 ft. 5500 + 120 = PH 5,620ft

c) (-5 x 120) + 5,620 = -600 + 5620 = 5020 ft DH

14th Apr 2002, 18:11
for question 1:i think in some specific situation, increase of clearway could have a little bit effects on MTOW. bec the MTOW also limited by climb gradient, so if the clearway extended long enough, then climb gradient reguirement will be decrease,so the MTOW could be increase in some cases.

Empty Cruise
14th Apr 2002, 19:14
Can increased clearway increase TOM?

Yes, if you are accelerate-go-limited. But, as stated above, the perticular AD, surrounding terrain & A/C charachteristics will put a limit on how much TOM could increase. ;)

What will effect on V1 be if clearway increases

Assuming that the clearway cannot simultaneously be used as a stopway (LAX example), the aircraft will maintain the same V1 for the accelerate-stop scenario, i.e. the V1 cannot be any higher.

You could lower the V1, as the aircraft would have more distance over which to accelerate to V2 while climbing to 35 ft. - but what would be your advantage? None, unless you also alter the TOM. In the case of clearway, you would have the option of:

1: Selecting a new V2, higher than V2min. This will allow for improved climb.

2: Adding payload or fuel to the aircraft, thus increasing mass. The V1 will be lower, as the aircraft will be harder to stop & thus require a lower abort speed. Due to the increase in clearway, the aircraft will be able to reach V2min at 35 ft., even with the lower V1 and increased mass.

As you see, the gain in TOM would be rather low, as you will have both increased mass and lower V1 eating away at your increased clearway.

I...errr...think... :D :D :D

15th Apr 2002, 02:47
... probably because there are very few definite, right answers to many of these sorts of questions ...

1. Clearway.

Increased clearway generally is of use if

(a) it doesn't extend beyond the limiting TOD/TOR considerations .. i.e. there is a limit to the amount of clearway (included in TODA) able to be used before the TORR becomes limiting or, if you prefer, there is a limit to the proportion of the airborne flare distance to 35 ft which can be over the clearway. This limitation is to give you some confidence that you will be off the ground before you run out of hard surface. As a result, a very lengthy declared clearway often is only partly usable for a particular aircraft.

(b) any resulting increase in RTOW (or MTOW if you prefer) doesn't push the net flight path further out sufficiently to cause conflict with terrain which it missed at a lower weight

(c) on another tack, if RTOW is not a pressing problem, you may be able to use some more clearway, subject to (a), to permit a reduction in V1 and provide some (more) ASD stopping margin.

If the aim is to maximise RTOW, then one would normally push the V1 up to achieve (or approach) an ASD limited takeoff. This minimises the V1 to VR acceleration distance so that the clearway can best be used to get a bit more RTOW. Definitely a case of juggling the numbers to get the best compromise result .. as the particular case which provides the lowest takeoff weight becomes the limit for the particular conditions there is no point in pushing one limit weight up if another goes down in consequence.

If the aim is to get some extra ASD margin, then one would reduce the V1 until either TORR or TODR becomes limiting.

Generally, there would be not much point in keeping V1 the same as the previous value has already given a limit case for one of the various calculations .. with the same V1, the takeoff doesn't vary unless the TODR was the significantly limiting case. In this situation, you might get an improvement in RTOW at the same V1 as there would be some ASD, TOR, and (with the additional clearway) TOD available with which to play.

I think Empty Cruise is assuming a limiting ASDR and has overlooked TORR ?

2. Slats and flaps.

Slats and LE flaps tend to extend the basic wing CL vs alpha curve to higher alpha and CL. Trailing edge flaps tend to shift the basic curve to a higher CL range. With both on board, the effect is cumulative. The answer to your question is, I suggest, probably not answered simply as one has to think about, in addition to the body angle, the flight path angle, the changed airflow patterns due to the devices, and where the wing sees the wind coming from as a result (alpha) .. a fairly dynamic situation. I shall be interested to see what answers other people might come up with during the life of this thread.

3. Density height simplified calculations.

A reasonably accurate calculation is -

DH = PH +/- (120 * ISA deviation in deg C)

As the equations are straightforward to program into a spreadsheet once the constants are put in, you might like to play with them on your PC. For the troposphere, the basics are

(a) temperature in Kelvin (K) = 273.15 + OAT (C)

(b) ISA temperature (K) = 288.15 - 0.0019812 * height (ft)

(c) temperature ratio = theta = OAT (K)/288.15

(d) pressure ratio = delta = theta^5.25588

(e) density ratio = sigma = theta^4.25588

Using the non-dimensional quantities, theta, delta and sigma makes the sums easier and, for the latter two, we avoid having to work with dreadful numbers

(f) having picked a height, calculate the standard temperature, theta, delta, and sigma. These are the figures you will find in ISA tables. If you wanted to know the actual pressure and density values, you could multiply the non-dimensional values by the sea level values ... not much point in that, though.

(g) to allow for a non-standard temperature, the general relationship is

sigma = delta/theta

(h) so calculate theta for the non-standard condition (c) and then sigma (g). Delta is standard as we are considering the situation on 1013 mb with a non-standard temperature only. As has been suggested earlier, if the QNH is not 1013 then a correction needs to be made to bring us back to standard pressure conditions.

(i) then go back to the standard atmosphere equation for sigma (e) and figure a standard height which has the same sigma as calculated in (h). This calculated standard height is referred to as the density height .. ie the height in ISA where the actual density (or density ratio) is located.

Having always just used 120 ft/deg as the correction, which I read somewhere when I was learning to fly, I thought it would be fun to do the above and check it out. Looking at a few heights from sea level up to 15000 ft and deviations from ISA of up to +/- 30 C, (and assuming I haven't made any careless mistakes), the correction rates vary non-linearly but within a small band ranging between about 110 and 130 ft/deg. Looking at a straight line curve fit for constant heights, the average correction is around 119 ft/deg, so call it 120 ft/deg to make the mental arithmetic reasonable... any error is not so great as to have any operational significance.

15th Apr 2002, 20:27
Q2 of Quirrels post only mentioned maintaining a constant attitude. Therefore the increase in CL and AoA from hi-lift devices deployment will also result in a decrease in IAS/EAS due to inducement drag (CD) increase, assuming present thrust remains constant throughout. Graces question didnt say anything about wether the attitude is in a climb or cruise or descent right-side up or upside-down.

John Tulla while I got you just 2 questions:

How much faith would you put in the latest Boeing TO and LDG performance tables (now a part of the QRH) as against the TO and LDG performance graphs that were part of the old manuals? For 737-300 I extract diferent data for the same conditions from each of these performance referances even though each respective datum in the end-calculation should be the same. (I retained the old graphs and didnt chuck them out).

As most Boeing drivers know, the QRH-referanced LDG dist data tables only gives you expected runway length to be used, and is the total of:

* 50 ft height to 1000 ft markers (touchdown) in 5 secs.

* 1000 ft touchdown to point of maximum manual braking fully applyed - 800 ft/3 secs

* From max braking to full stop.

There is no 1.67 factoring, nor a 1.15 additive for wet runway on data derived from these tables.

In terms of LEGALITY would these performance data tables in the QRH satisfy a court, or would the figures derived from the more accurate graphs found in the aircraft's AFM be over-riding? [a good example would be the restricted MLW for ANTISKID INOP, WET RUNWAY with limiting LDA. 100 ft can make all the diference.] Thanks mate.

16th Apr 2002, 01:02

I would need to play with the specific sets of QRH data and the relevant AFM data to form a view.

If you can see a difference between earlier and present QRH data which, on the surface, you would have expected to be the same, then there is likely to be a difference in the calculation basis for the data.

I would be extremely cautious using the QRH unfactored landing data .. not because they are suspect at all (I am sure that they can be tracked back to the same sorts of numbers in the AFM) .. but that they are based on particular assumptions and represent distances (for those assumed conditions) which may be extremely difficult to reproduce in line operations.

The line pilot who uses the QRH data for in-flight decision making without considering the normal fudge factors is a brave man .... I far prefer to view such data as representing an in-flight planning absolute minimum for emergency situations. I would not be at all comfortable to view such data as representing the performance which might be "expected" on the occasion.

The certification data is the definitive set of numbers and, for the most "accurate" data, one would refer to use the Boeing performance programs for the particular airframe/engine/configuration in preference to the paper AFM or, if the computer is not available, the paper AFM. The QRH is, after all, only a hip pocket pilot help document .. even if the document continues to get bigger and bigger ...

As to legality ? That is the final view of the judges in the appropriate jurisdiction after the accident .. or the regulator in respect of AOC audit action .... I would suggest that, for the line pilot, the main aim is to have acted in a conservative manner in line with company OM guidance ... considering and as appropriate to the particular circumstances ... and be able to justify his or her decisions and actions against whatever criteria pertain for a particular jurisdiction. One's company ought to have taken suitable legal advice on any contentious matters when considering what guidance to provide in the OM.

16th Apr 2002, 06:30

We are not sure why Boeing changed their philosophy, but it appears to be post SR111 and applies to more than just the B737.

The QRH data is giving you test pilot stuff for when you HAVE to land your aircraft immediately, use them at your peril for normal operations. You would be a lot better off sticking with your old graphs that presented a pre-departure “Landing Distance Required”.



16th Apr 2002, 07:38
Slasher and Mutt,

The 737 QRH I was using last year had unfactored landing data and I presumed that Slasher was referring to a difference between that earlier data and something which has come out in recent months ? Mutt appears to be contrasting factored and unfactored ? ... or am I just confused ... ? If, in fact, the change relates to a change in philosophy post accident, then I guess I may have been using the same document (or similar) to which Slasher refers.

I would still urge considerable caution in the use of QRH unfactored data. This will have been generated for the nominated conditions using the aircraft performance model and will be untested. I would be surprised if it contains any fat at all.

What I have used as a teaching philosophy in this area (assuming that the operational situation presents a little bit of time) is something along the lines of the pilot doing some initial sums on the basis of 1.43 (cf alternate landing in some jurisdictions - rounded off to 1.5 for convenience) and 1.67 (cf normal - rounded off to 1.7). As part of his/her risk assessment a runway under consideration can be categorised as "normal" (>1.67), requiring considerable care (1.67 - 1.43), requiring extreme care and precision in the landing (1.43 - 1.00), and guaranteeing an overrun (<1.00). If the runway is wet, then the 1.15 type of factor can be included to suit. (I suggest that the idea of undershooting to pick up some extra ground distance is fraught with danger).

If there is only one feasible alternative for recovering the aircraft, then the above reduces to an assessment of the degree of care required for the landing and emergency facilities pre-arrival briefing.

If there are no options at all and no time .. then give it your best shot and hope that Lady Luck is with you ...

Empty Cruise
16th Apr 2002, 18:00
Hi John T!

Actually, was assuming initial condition ADSA=TORA=TODA. Then simply adding a piece of land qualifying as clearway, but not stopway. So I got to ASDA < TODA > TORA. Therefore, I tried only to answer the part relating to increasing clearway above zero, trying to avoid confusing the question with how much clearway you can use. ;)

Makes senes or not:confused: :confused: :confused: ?

Empty CRS

17th Apr 2002, 05:35
John T (and Mutt) thanks for that. Sorry, the AFM graphs and the old manual graphs can be taken as one in the same for the purpose of my question.

Yeh we primarily use the old landing graphs for landing-dist calculations and use the QRH-distance tables as a back up ("what distance I legaly require" vs "what distance Boeing expects me to use"). As you say there is no fudge-factoring in QRH-derived distances. But increasingley I hear 737 pilots using the QRH-ref distance tables as the sole source of LDR, even though these tables are headed ADVISORY INFORMATION. The old graphs are not "advisory" in the legal sense. This not only applys to non-normal configuration distances but normal distances too.

Still the question remains: will the sole use of QRH-derived LDRs stand-up legaly at the Tribunal and/or the Beak after the over-run? The implication here is that Boeing made the old graphs redundant (please chuck out and replace with....) but the AFM graphs (almost the same as the old graphs) of course remain over-riding.

17th Apr 2002, 07:30
Oh ... oh ... let's not be too cavalier there, good Slasher ....

(a) the AFM as approved or authorised for use by YOUR local airworthiness authority (and one needs to consider the State of Registry as well as the Authority which controls your company's AOC) dictates what is the minimum acceptable airworthiness data. The operations people or, indeed, your own people (but the latter is probably unlikely), might well impose some additional requirements for some or all operations and I suggest the final requirement is what is the legally required minimum) .

If you don't have the distance required by the AFM and operational requirements which sit on top of that document then you probably are going to find yourself VERY, VERY lonely at the inquiry. The company is unlikely to support you at all ... nor would I like to have tea and bikkies with the insurance people either ...

(b) I suggest ... that the unfactored QRH tables are ABSOLUTELY ADVISORY and only give you an INDICATION of the minimum distance you MIGHT be able to get away with ON A GOOD DAY in an emergency where you don't have a better alternative. I think that you are taking a gigantic leap of misplaced faith to even think that Boeing is suggesting that the QRH data is what you are "expected" to use. I have no doubt at all that Mr Boeing would direct you to the AFM and any over-riding operational requirements associated with your AOC.

In using the QRH data without lots of extra distance (ie at least what the AFM says you require and hence my suggestion to my students to look at the normal factors on top of the QRH data), I would feel a very icy cold wind whistling in and about and around my bewtocks (as Torch would put it in a well known magazine) ....... think of it like there is a BIG brick wall at or about the distance suggested in the QRH ... for this is something in the vicinity of what it means ...

If you do a search on this subject, there was a thread some time ago wherein a chap from the frozen north related similar tales ... he acknowledged that ill-considered use of the QRH data .... provided regular episodes of adrenalin-enhanced cardiovascular and alimentary excitation ....

If you are flying for whom I think you are, who is the Chief now ? He who was there earlier would never have condoned what you are suggesting is the norm .... in a fit.

Grace Quirrel
17th Apr 2002, 18:45
Thankyou all for your informed replies, the mud in the water has cleared.


17th Apr 2002, 21:24
Whoh hang on John your preachin to the converted mate! If you go over my last post again youll see Im a staunch advocate of distances derived from the AFM, and the QRH distances as a back-up only to AFM distances (Ex: AFM says I legaly required 6000 ft of runway on the day, QRH says Ill probabley use about 4730 ft of it in landing and stoppin the damn thing). In my last post I meant what distance of runway Boeing expects me to use (actualy chew-up) on the day, not what primary method of distance-derivation Boeing expects me to use. I meant it in that interpretation.

However I think youve answered my original question anyway.

After all that I think its time for a beer!

18th Apr 2002, 01:47
Grace Quirrel,

Hopefully our errant off thread ramblings are not too distracting ...


A bit confused here without having in front of me the particular docs to which you refer... is the 6000/4730, which represents around a 25% penalty, from the manual/QRH for the same conditions ?

My concern with your post is that, if the QRH refers to unfactored distances, then one ought to interpret that to mean something along the lines of "in the conditions specified, and you're otherwise having a good day, if you do everything right, don't float, use maximum effort braking, and are reasonably lucky .. then you might go close to achieving the QRH distance .. but don't stake your life on it".

Subject to what the present QRH says (and mine might well be a revision or two out of date), and how it relates to the AFM, I would be very surprised indeed if Boeing is suggesting at all that the QRH figures bear any resemblance to what the line pilot will achieve in a routine landing.... That is why the certification factor of 1.67 is there in the first place .. to provide some fat for the normal variability we see in the landing manoeuvre. When you consider how the performance model is developed and, more importantly, validated by flight test, then one ought to be very cautious when the manufacturer makes reference to unfactored distances ...

If you are able to do so, I would dearly love to see a scan of the two sets of data as that would make it much easier for me to get my head around how you are viewing the situation.

.. and the beer is a foregone conclusion ...

18th Apr 2002, 03:16
If I assume correctly judging by your registration date you're fairly new at this (no offence) and you are looking for a text book JAA/CAA answer (if not just ignore me).

With reference to Q2 Capt Pitbull is absolutely correct but just to put it in terms of exam questions.

At a constant attitude Lowering the flaps increases the angle of attack of the wing. Extending the slats decreases it.

It might be worth noting that the aerodynamic chord is the straight line between the forward tip of leading edge of the wing and the tip of the rearmost trailing edge. So if you change the position of the leading/trailing edge the chord must change. Sketch it out to see how.

As far as density alt is concerned I have read and been told the best way to calculate it is on the flight computer (crappy-5/E6B) so that's how I do it. It's only a 30 sec job after all.

18th Apr 2002, 03:44

The whizz wheel gives you a quick DH, but the scale and usability of the device is such that the answer is considerably rougher than doing the 120ft/deg in the head calculation. Then again, the accuracy required and appropriate depends on what you want to do with the answer.