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Smoked Toilet
11th Apr 2009, 22:56
In a CRJ, only pressure altitude and weight are used when determining actual landing distances. Why would Bombardier not use Density altitude, or add in a correction for temperature deviation?

Thanks,

S-T



edited for "actual landing distance"

Mad (Flt) Scientist
11th Apr 2009, 23:11
???

There are temperature inputs in many of the charts in Chapter 6, Performance, of the Airplane Flight Manual.

Para 3A (06-03-125) provides the instructions for TODR-OEI and ASDR. One of the listed values required is Temperature. Para 3B is the AEO TODR case, and again directs the use of temperature.

Its a bit complex because it's spread across several charts, with the use of reference values 'A' and 'B' to avoid needing a huge fold-out chart on one page, but it is there if you follow it through.

Smoked Toilet
11th Apr 2009, 23:17
Ill have to look next time Im in the airplane, but our comany Actual Landing Distance charts do not use temp. Also, we do our landing performace via ACARS, and temp is not inlcuded in those calculations. (There is a spot to input it, but you can change it from -40 to +40 and it wont change the numbers at all)




edited to remove takeoff

Mad (Flt) Scientist
11th Apr 2009, 23:19
I only checked the takeoff info.

There's definitely no way you should be getting no change in TO perf from +40 to -40 - the engines aren't flat rated anywhere near that high, and at +40 you should be seeing significant TO perf degradation. Unless you're flexing like mad on every TO?

Smoked Toilet
11th Apr 2009, 23:23
We do flex on most takeoffs, but that has no difference - we get both normal and reduced thrust numbers on the same page. For example, on the landing distance data page, it requires the following inputs:

Weight
Airport code
Rwy

Wind is optional.

Then temp and pressure will autofill if the airport publishes a METAR or D-ATIS. You can manually input these and override the autofill, and pressure will adjust the data it returns. However, when you adjust the temperature, nothing changes.

mutt
12th Apr 2009, 04:56
You should really talk to your ACARS performance supplier as they are giving you crap. We analyzed the CRJ700 takeoff and landing weights with a view to purchase, it was extremely evident that there is a weight decrement for takeoff and landing associated with temperature.......

Mutt

BOAC
12th Apr 2009, 08:55
"Wind is optional."....................:eek:

Smoked Toilet
13th Apr 2009, 01:15
Yes, wind is optional. In other words, you can plan on zero wind, or you can include wind for your calculations - since wind is seldom a constant. Not implying that dismissing wind is a good idea.

I checked the AFM. The AFM Performance section landing charts don't include temperature either.

Anyone?

john_tullamarine
13th Apr 2009, 23:10
Temperature will be included in the takeoff calculations as, for instance, required by FAR 25.105 (http://ecfr.gpoaccess.gov/cgi/t/text/text-idx?c=ecfr&sid=405acdaaa8d81161425eaa4a2d29ec17&rgn=div8&view=text&node=14:1.0.1.3.11.2.155.10&idno=14). However, it is not uncommon to find no temperature provision for landing as, for instance, seen in FAR 25.125 (http://ecfr.gpoaccess.gov/cgi/t/text/text-idx?c=ecfr&sid=405acdaaa8d81161425eaa4a2d29ec17&rgn=div8&view=text&node=14:1.0.1.3.11.2.155.20&idno=14).

If your AFM appears not to include temperature for takeoff then, as MFS suggests, it may be a case of its not being as obvious as you might like.

There is nothing to preclude the inclusion of temperature as a variable in the landing data and, sensibly, one is going to make an adjustment for non-standard conditions even if only to use density height in lieu of pressure height as the argument.

Old Smokey
14th Apr 2009, 00:27
Refer to Mutt's post, you are being served a heap of crap!:mad:

Just a few contentious points to bolster various remarks made here -

Density Height is commonly used for Piston Engined aircraft data, but NEVER for Jet operations, there's several reasons why.

Density is a direct product of Temperature AND Pressure, the provision of these two suffices for those instances where Density is important. Thus, if Temperature and Pressure are quoted, there's no need to refer directly to Density.

Wing Lift and other aerodynamic forces depend upon density, but T & P together will give you that.

Calibrated Airspeed, Mach Number, and CAS/Mach crossover depend upon Pressure alone without reference to Density or Temperature.

Engine thrust depends upon density (mass flow), but T & P together will give you that.

Jet Engines are either Temperature or Pressure limited in isolation to Density, thus knowing Density alone is not enough. (They're also speed limited, but that's not being discussed here).

Summary - Knowledge of Temperature and Pressure in isolation is VITAL, in those cases where Density is important, the knowledge of Temperature and Pressure in combination suffices for the calculation.

To comment on John_Tullamarine's remark that Temperature need not be considered for landing calculations, whilst I prefer to consider it, in the preparation of General Landing Charts, the Temperature correction is quite small, and the manufacturer could be forgiven for assuming the worst case environmental envelope temperature. Landing is NOT Thrust dependant, whereas Takeoff IS, and very significant Takeoff Performance variations are evident with temperature variation.

BOAC's comments regarding "Wot, no wind?" are acceptable so long as we are considering zero wind or a Head Wind. Presumably the aircraft is not certified for Down Wind Takeoffs?:confused:

Regards,

Old Smokey

john_tullamarine
14th Apr 2009, 01:08
A couple of observations ..

but NEVER for Jet operations

.. but may be used for landing distance adjustments where we are looking at a flight idle situation for which thrust is not a pivotal concern.

.. temperature need not be considered for landing calculations ...
the manufacturer could be forgiven for assuming the worst case environmental envelope temperature

the usual certification requirement is standard conditions (ie ISA) for landing and this is the usual AFM presentation. Typical operator adjustment is to use DH in lieu of PH.

mutt
14th Apr 2009, 05:34
Kind gentlemen, if i was receiving a limiting landing weight through ACARS, i would expect it to account for approach and landing climb, both of which are influenced by temperature.

Mutt

john_tullamarine
14th Apr 2009, 05:38
.. and so it is (or should be) .. only the landing distance may be calculated for standard conditions. The limiting landing weight then would be the most limiting of the three weights .. as per normal protocols ..

... but you knew that all along ...

BOAC
14th Apr 2009, 07:37
so long as we are considering zero wind or a Head Wind - indeed, but is that not in a way saying "Wind is NOT (strictly) optional."? That must be in the POH somewhere, surely?

Regarding LDR and temperature, it is not clear whether ST refers to Perf MLM or LDR when talking about "The AFM Performance section landing charts". I too would expect Temp in the MLM.- indeed, any perf item which is thrust related. Likewise in LDR to cater for higher TAS/energy with ISA+.

john_tullamarine
14th Apr 2009, 08:15
is that not in a way saying "Wind is NOT (strictly) optional."? That must be in the POH somewhere, surely?

One needs to distinguish between what the Design Standards require to be in the AFM (by whatever title) and how the operator might provide such data to line crews.

If we consider takeoff and landing distances (for US certificated Types - but the philosophy is pretty general) then 25.105 and 25.125 impose a must condition for wind scheduling. ie the operator is stuck with the AFM wind carpet corrections .. you must not takeoff at a greater than AFM tailwind nor can you take RTOW credit for greater than AFM headwind.

The operator, on the other hand, might well simplify the AFM data in the local operating manual along the lines which OS has suggested .. so long as the operating manual data is conservative with respect to the AFM requirements. This is a pretty typical sort of approach to establishing general takeoff charts for line use .. to run an AFM calculation generally is not viable within the constraints of a commercial schedule.

Regarding LDR and temperature, it is not clear whether ST refers to Perf MLM or LDR when talking about "The AFM Performance section landing charts".

Generally, US AFMs will not include temperature (other than the inferred standard temperature for Hp) for landing distance. Some countries will vary this - eg Australia always used to require the Australian AFM to schedule Hd rather than Hp for just this concern.

However, when determining MLW (or MLM) you still need to check the WAT limits and they have a temperature accountability .. as is technically sensible.

MLM and LDR are just two ways of looking at the same thing for a distance limited landing. One needs to keep in mind that, for all these calculations, one runs a bunch of sums for various potentially limiting requirements .. with the most limiting resulting value becoming the MLM.

I too would expect Temp in the MLM.- indeed, any perf item which is thrust related.

For landing distance, the thrust is flight idle so can be sensibly ignored... but not so for the approach and landing WAT limits.

Likewise in LDR to cater for higher TAS/energy with ISA+.

An oft-cited concern .. which is covered if you use Hd rather than Hp.

Just one of those certification quirks, I'm afraid.... and one of the reasons Tech Log has such an important educational role for the flying community .. to cover all those little things which either got glossed over in ATPL theory .. or weren't even considered.


Note in contrition .. I have been chastised for jargon ..

Hp = PH = pressure height (or altitude)
Hd = DH = density height (or altitude)

.. sometimes I get a tad slack, I know ...

gengis
14th Apr 2009, 09:16
In my airplane (-400) the scheduling of Takeoff Performance is factored by ISA deviation.

This takes care of the temperature question

Smoked Toilet
15th Apr 2009, 01:48
I appreciate everyones professional responses. I apologize for not being more clear in the beginning of this thread (I have since gone back and edited the above posts to try and be more clear). I am concerned about Actual Landing Distance Required - Not any maximum landing weight that might be influenced due to obstructions etc. Just "wheels on the pavement" distance.

In regards to Old Smokey, we include pressure in the calculations. As you change the pressure setting, ACARS does send back data reflecting these changes. However, when we adjust the temperature input, nothing changes. In other words, there are no "density" calculations being made.

I still don't quite understand aerodynamically why temperature would not influence (or for that matter, why density is not being considered) the landing distance.

Take care,

S.T.

Mad (Flt) Scientist
15th Apr 2009, 02:19
I still don't quite understand aerodynamically why temperature would not influence (or for that matter, why density is not being considered) the landing distance.

OK, there's two different questions here, and it's an exercise in pedantry to separate them, but I will.

The answer is that in the real world there is a temperature/density effect on the actual amount of runway you need to land, but the requirement for actual landing distance to be determined is regulatory in nature, and ever since Part 25 was first released in 1964 the relevant words have read:

Sec. 25.125

Landing.

[(a) The horizontal distance necessary to land and to come to a complete stop (or to a speed of approximately 3 knots for water landings) from a point 50 feet above the landing surface must be determined (for standard temperatures, at each weight, altitude, and wind within the operational limits established by the applicant for the airplane):
(My emphasis)

So all I am required to do, as a Part 25 OEM, is determine (and publish) landing distance data for ISA. I'm allowed, legally, to ignore the second order effect on actual landing distance due to temperature.

And all an operator is required to do, legally, is operate to the approved data associated with the Part 25 aircraft.
121.195 Airplanes: Turbine engine powered: Landing limitations: Destination airports.

(b) Except as provided in paragraph (c), (d), or (e) of this section, no person operating a turbine engine powered airplane may take off that airplane unless its weight on arrival, allowing for normal consumption of fuel and oil in flight (in accordance with the landing distance set forth in the Airplane Flight Manual for the elevation of the destination airport and the wind conditions anticipated there at the time of landing), would allow a full stop landing at the intended destination airport within 60 percent of the effective length of each runway described below from a point 50 feet above the intersection of the obstruction clearance plane and the runway.
(again my emphasis)

Legally, we can produce ALD and LFL excluding temperature effects, and you can use them as gospel.

And it works. This is one of those cases where the extra effort to provide the exact answer doesn't seem to make the world any more or less safe - and that 60% factor between ALD and LFL is to blame (or thank, if you like). There are so many other variables in the "landing distance of the day" compared to ALD that a bit of temperature deviation gets lost in the noise. A bit too much float, a bit late on the brakes, a bit more rubber on the runway and it's neither here nor there.

After all, your WET LFL is utter nonsense, scientifically speaking. Only 15% more distance to stop, wet vs dry? Reality is probably more like 30-40% - but if the world applied a 33% LFL wet factor, say, half the world's air routes would only be viable when the sun was shining....

just to note that J_T covered a lot of that in terms of the regulations, just didnt quote specific regs

Smoked Toilet
15th Apr 2009, 02:30
Mad (Flt) Scientist - I thank you for your time. This was a great discussion; I appreciate you sharing your knowledge.

This was my first thread on PPRUNE, and I appreciate everyone chiming in to help me come to this conclusion.

Fly Safe,

ST