RTOM "flight path" limit
 

Hi,

Could someone point me to a good resource, or in the right direction, that would help me understand an increasing RTOM with increasing temperature for a given runway. Limiting code is given as FP, which stands for "flight path/level-off altitude limit (T/O thrust limit)" in our third party performance data.

We have seven zero-wind runway limit codes: structural, field length, obstacle, tyre speed, brake energy, min control speed and "flight path/level-off altitude limit (T/O thrust limit)". The airfield in question has a 2500 feet cleared altitude on departure. I'm presuming that this is what is influencing the restricted take-off mass?

I have been poking around here, Smart Cockpit and Skybrary but so far to no avail.

Thanks in advance.

Bit of a stab in the dark without more info, but some airfields have a minimum climb gradient to achieve a certain height by distance from start of the T/O roll, so consequently an increase in temperature will limit the amount of mass that can be lifted to that height because of reduced performance

Hi,

The RTOM increases with increasing temperature. Every other limiting case I've seen has a decreasing RTOM with increasing temperature i.e. reducing performance.

A bit more info may help direct the investigation:

1. Is it a large or small effect? (A small effect may simply be due to rounding/analysis errors and not truly a physical effect)
2. is it happening at all temperatures, or only in a range? How does that range 9if that's the case0 compare to, say, engine flat rating temperature?
3. (IT Help desk "is it switched on?" question:)) is anything else varying at the same time, other than DIRECTLY due to the mass (config, thrust, etc.). I assume not, but "never assume"

RTOM around 41000lbs increasing by a couple of hundred pounds per 2C temp. I think level off at the airfield is 2500 feet. Was just looking for examples of field length limited RTOMs in system and this FP limited example came up.

"flight path/level-off altitude limit (T/O thrust limit)" is the limit which is set to occur to the maximum light path/level-off altitude which will enable the engine limitation to be observed to takeoff, e.g. in case of 737 the default for this is 5 minutes or the optional 10 minutes could have been purchased.

If a takeoff is performed at a thrust setting e.g. 22K at an airfield and you experience a EFTO at or near V1 thus continue the takeoff, the weight at which the aircraft can climb to MFRA, accelerate to flaps up and climb to >1500' AGL within 5 minutes is used to calculate this limit.

In performance software the administrators are capable (by manipulating the settings file) to provide this sort of information to flight-crew by displaying minimum and maximum MFRA values. Minimum MFRA will be used by crew for single engine flap retraction, maximum MFRA indicates the altitude at which in such case acceleration must have occurred in order to not bust the engine limit. The larger the gap between the two values, the more time is available. The gap is reduced for a given thrust rating when ATM is applied.

If I understand correctly the confusion seems to come from the increase in allowable weight with increasing temperature; where as it is normally the opposite. Is that correct?

I would have thought your company performance engineer and data supplier would be the first stop on your quest for the truth. Could it even be a mis-print? Crazier things have happened.
From your weight (41000lbs) I assume this is USA =FAA and a turbo prop or small commuter jet. Under what rules are you operating and who is your performance data supplier?

We would be very interested in the answer to the query.

Hi Rat 5,

Yes, the confusion is the increase in RTOM for increasing temperature. I understand the situation Skyjob describes, but in the case they put forward, I would expect to see a reducing RTOM for increasing temperature as performance decreases. Plus, with the example given, the level off altitude is 2,500 ft (out of Northolt if that is pertinent).

Aircraft is a Falcon 2000 under EASA Annex IV Subpart C, in particular Performance A (CAT.POL.A.200-250).

APG is the provider. Whilst our Ops Manual does reference and describe the restricting factor (FP), it's transplanted directly from APG's guidance material which I have and is no less explanatory. I'll give APG a go. However I won't hold my breath as I have contacted them previously concerning the OEI Departure Procedure and never received an acknowledgement or response.

One benefit of never being the smartest guy in the room is that forums like this often yield an answer when my limited mental capacity is exceeded! :}

Haven't seen this in my own ops eng work in the past.

Speculation only, but I harbour a suspicion that it will be tied up with takeoff optimisation goings on in the (software) background - which are not explicitly declared - but probably involve clever juggling of thrust reductions across the MTOM chart data ?

Alternatively a looksee at the temp limited thrust chart might provide a clue ...

OK, kicked this around a bit and solicited some ideas. How about this?

One of the few things which gets "better" with increasing temperature is VMC - if you are scheduling it with temp, it goes down as temp increases (due to the reduced thrust). So, maybe it's a VMC effect?

Now, I'd usually think that the direct effect of the thrust loss would win; all the VMC scheduling does is reduce the impact of the temperature, but not actually improve things overall. But suppose the aircraft were heavily stop-limited. Then you'd want as low a V1 as you can get, and maybe that drives the balance such that a higher temp, with a (much) lower VMCG, wins out over the reduced T/W?

In-air, if you had an unreasonably high VMCA which was driving adversely high speeds, maybe a similar effect would exist? Though this case implies that the aircraft would be better off simply derating, if more thrust is bad, so it sounds a bit theoretical.

Anyway, I offer up VMC as possibly part of the explanation....