Obstacle assessment for takeoff

[UIWM 1]

Hi there,

 

Takeoff obstacles is an important implication on performance capability. For years, flight sim community in general was V-speeds centric so to speak, will little attention paid to obstacles. Consequently, we have a lot of tools that determine other takeoff parameters but completely ignore assessment of takeoff flight path.

So I would like to ask you how do you approach this problem on your routine flights what are the techniques you use, specifically how do you determine the appropriate obstacles (1) and how you assess your aircraft's performance in this regard (e.g. limiting takeoff weight) (2). Question includes both all engines operation and one engine out configuration, so a side question is how do you come up with engine-out escape route/EOSID (2-A). My question is for simulated commercial operations under IFR.

Personally, when I used to use PMDG B737, TOPCAT did a job, but I did find it either too limiting or inaccurate in difficult cases. Also, obstacles database it uses (even if updated per airac) sometimes significantly differ from the data from airfields' AIP, requiring manual entry. Another issue is that is gives EOSIDs, yet these are generic EOSIDs without a note to which aircraft they apply, effectively rendering them useless.

 

My question may seem trivial to people who mainly operate on plains, but consider this: suppose you are to depart from Sion, Switzerland, LSGS. One of the SIDs for runway 25 requires terrain-limited gradient of in excess of 13%. How would you determine whether your aircraft is capable of it?

 

PS. This topic is for simulated environment, assuming that there is no access to real world RTOW tables, OPT software, etc.

Thank you.

 

[Wolf0 31]

Unless you have access to professional performance software the answer is: 99.8% of the world simulated flights are conducted without accounting for terrain clearance.

There's no tools available to compute that. The closest is indeed topcat with all it's limitations.

I guess our virtual passengers aren't all that safe (especially now with GSX lvl 2 we can actually see who we'll be killing in case of a failed engine climbing out of Innsbruck in IMC)!

Jokes aside I'd be willing to pay for such software, but commercially speaking I guess the market isn't that big. There's a significant part of the community that values VFR and the ones who simulate airline procedures do that with different depths of realism... After all, if topcat was a great success it wouldn't be abandoned as it is now...

[JRBarrett 3,072]

It can be done “the old fashioned way”, using pencil and paper, as it was many years ago before the advent of computers.

Most departure procedures in the U.S. will specify a minimum climb gradient expressed in feet gained per nautical mile when terrain is a concern. The runway 24 departure at my home airport KELM, calls for “405 feet per NM to 2300”

Using the Aerosoft CRJ as an example, I would typically maintain 160 knots IAS during the initial climb segment until reaching flap acceleration height. 

Dividing 160 by 60 gives a distance covered of 2.66 NM per minute at that IAS. That’s in a no-wind situation, but any headwind (as would be typical) works in favor of the aircraft.

Mutiplying the distance covered in one minute (2.66 NM), by the required climb gradient (405 feet per NM) gives the required vertical speed of 1077 feet-per-minute. As long as the pilot maintains 160 knots IAS and a minimum climb of 1077 FPM until reaching 2,300 feet MSL, then terrain clearance will be assured on this particular departure.

The limiting constraint is 2,300 feet MSL. Once the aircraft reaches that height, it is no longer necessary to maintain a specific IAS or minimum rate of climb. In this example, most any jet transport aircraft will climb much faster than 1,077 FPM in the initial segment, so the minimum safe altitude will be reached in much less than one minute after takeoff.

Aircraft performance becomes much more of a concern with departures from high altitude fields like KASE in summer, where the calculated vertical speed might not be “assured” without consulting aircraft performance tables.

[Chock 17,769]

In the real world, airline crews have all this stuff calculated for them for each runway and the various weather and runway conditions, and that information is readily available to them. Plus, the airlines should, if they are doing things correctly, never plan to have an airliner overweight to the point that it couldn't make a required climb even if it lost an engine. So it isn't unrealistic to not go into it too deeply on your simulated airliner providing you are operating something which might reasonably fly off the runway you chose and is at a fairly realistic take off weight. But, as noted on some previous answers on this thread, and as you point out also, you can work it out manually. A useful web page for this is here, for anyone curious about the correct considerations:

http://expertaviator.com/2011/02/03/departure-procedure-climb-gradient-and-calculating-your-rate-of-climb/

Having said that, one of the things which should normally be either briefed, or at least reasonably clearly understood by pilots, is to ensure that in spite of all this planning, they should also always have a plan for if things go horribly wrong and they cannot make a required climb or turn after departure, even if that means bellying the thing into a field straight ahead if that happens to be the best option available under the circumstances. I always think about that too, certainly in real world flights, but also in simulated ones, even though it doesn't really matter too much in our sims if things go awry.

Any situation where problems such as that occur, is by definition a 'non normal condition' and in those circumstances, any SIDs, throttle back noise abatement requirements, or any of that other 'required' stuff can cheerfully go out of the window; the safety of the aircraft and its occupants becomes the priority. So at somewhere like Sion, whilst it wouldn't normally be the case that you could go whizzing down the valley at 500 feet AGL with the remaining engine(s) firewalled, if that's what it takes to ensure you don't get a face full of granite, that would be the right thing to do and you could argue about waking the neighbourhood when safely back on the ground lol.

In my real aeroplane flights, when getting ready to take off (and for several other checks), I add an E to the end of the memory check nemonic: For example, in the gliders which I've most often flown, there is the nemonic: CB-SIFT-CB, but, like many pilots, have amended it so it became: CB-SIFT-CBE (Controls, Ballast, Straps, Instruments, Flaps, Trim, Canopy, Brakes and Eventualities), i.e. I always take some time to think about this kind of stuff: 'what am I going to do if this, or that, happens on the take off. At what height/speed would I consider turning back to the field and landing back downwind, or landing ahead, or choosing some other option if the cable broke or some other problem occurred?' And so on.

I think it reinforces good habits to do this on our flight simulators in spite of it not really mattering too much in terms of mortal danger when flying a PC.

Edited September 10, 2018 by Chock

[tahdog 0]

Just a note to address an error that many make on departure procedures. The standard climb gradient for departures at airports that have had a diverse departure assessment done (any airport that has been terped to develop an instrument approach will also have a diverse departure assessment done) is 200 FPNM which must be maintained to the enroute environment, which is considered to be either the minimum altitude for IFR operations per FAR 91.177 if on an uncharted route, or the MEA if on an airway, or the altitude assigned by ATC. When a non-standard (increased) climb gradient (CG) is required, such as the departure from Rwy 24 out of KELM referenced by Mr. Barrett above, the increased CG must be accompanied by the MSL altitude to which the increased CG must be maintained to. However, once that MSL altitude is reached, you are still required to now maintain the minimum climb gradient (200 FPNM) beyond that listed MSL altitude until you then reach the enroute environment altitude as explained prior. The increased CG and corresponding MSL altitude are to ensure required obstacle clearance (ROC) for a specific obstacle in the departure area, whereas the standard 200 FPNM CG ensures ROC to the enroute environment. So, in fact, there is still a requirement to maintain a certain CG beyond the 2300' MSL in the departure procedure referred to above out of KELM in Elmira/Corning, New York. This is referenced in both the FAA Instrument Procedures Handbook and TERPS order 8260.46 c, Departure Procedures Program.

Edited January 16, 2019 by tahdog
added references.