Wednesday, August 10, 2011

Nerdy Flight Planning Question

I seem to spend about an hour a day making flight plans, maybe planning five flights for each one I depart on, because I have to be ready to go now for wherever the weather favours. You'd think after I got the one-way airways sorted out there wouldn't be much to plan, but I have to specify under instrument flight rules what we're going to do on a very non-standard flight. Instead of going from A to B by an efficient route, I'm going from A to a point that I must define precisely, but may depend on the location of the clouds on that day, then hang around in the same reserved airspace for hours, then fly to another arbitrary point to exit, and from there proceed to an airport of landing. An airport that is completely secondary to the purpose of the trip. We really don't care where we land, so long as it is long enough to take off again and they will sell us avgas.

Rather than specifying every photo line, we file "photo blocks" pre-named chunks of sky that you can see if you happen to have an old WAC chart lying around. Photo blocks have names like 093H4, and I file a list of photo blocks with the altitudes we need in each on a special flight plan form, then in the actual flight plan section I write (PHOTO BLOCK) instead of the name of an airway. I can enter the photo block at a conventionally named fix, and I will if there's one handy, but more likely we'll just be entering at an arbitrary point that I specify with latitude and longitude in the form 5100N11230W (ENT). There will be a second fix tagged (EXT) after the photo block, and everything else is like a normal IFR flight plan. Except we don't care where we land, but we still have to put something down.

On an IFR flight plan you specify your true airspeed in one block at the top and I'm used to putting down whatever fiction the manufacturer claims for the aircraft at the flight planned altitude, minus a bit for reality, and just leaving it. ATC knows that I'll climb a bit more slowly, and my initial descent will probably be a bit faster. At least that's how everyone I have flown with files and flies. But there is a mechanism by which you can specify speed changes, and any time you file an altitude change you have to refile your speed anyway, because the format is like this: N175A095 meaning 175 knots at 9,500'. But one day while listening to a departing Westjet flight being given a speed restriction in the climb, I thought about the fact most pilots don't climb into the flight levels by pulling their nose up to Vy with sustainable climb power and waiting patiently. What sufficed on an IFR flight plan down below might not be the best information for one at more rarefied atmospheric strata.

So I called IFR flight data and I asked them how an IFR flight that begins with twenty to thirty minutes of climbing at speeds significantly lower than the aircraft's normal TAS should be filed. The controller had an answer, but I wonder if it would be the same answer across the country and abroad.

What do you think the advice was? Or if you're a controller, what would you expect? I'll tell you what my guy said, in a couple of days.


Rob said...

I wasn't sure either and so looked up the Flight Plan section in the Australian AIP. It specifies both boxes (the top and the speed/altitude box) as cruise speed. So the 'book TAS +/- a bit' is the way to go here.

leisuresuitwally said...

I don't have my IFR rating, so take what I say with a grain of salt. According to the AIM (RAC 3.16.6) , you should put your "fictitious" speed in the CRUISING SPEED box, but then specify your changes in the ROUTE box.

(3) CHANGE OF SPEED OR LEVEL (maximum 21 characters):
The point at which a change of speed (5% TAS or 0.01 Mach or more) or a change of level is planned, expressed exactly as in (2), followed by an oblique stroke and both the cruising speed and the cruising level, expressed exactly as in (a) and (b), without a space between them, even when only one of these quantities will be changed.



Here's what Nav Canada's form looks like

The way I interpret this is that if you plan on climbing for twenty minutes, you should pull out your POH and convert your IAS to TAS based on METAR/TAF, and then figure out your cruise altitude for that rate of climb (ie 20 min at 500'/min = A100).

What I would probably do is write the climb TAS in the CRUISING SPEED box, then indicate my speed change to cruising speed in the ROUTE box (which seems counter-intuitive). I'd like to know what the controller told you.

Aviatrix said...

If you're going to be nerdy, you must know that the TAF does not forecast temperatures at all and the METAR reports the surface temperature at an aerodrome. For IAS/TAS conversions you're going to need the temperatures aloft from the FDs, but if anyone flying pistons does that kind of calculation to report their flight-planned TAS, they are nerdier than I am.

leisuresuitwally said...

Well... I guess I'm a big nerd. Then again, I don't calculate the TAS for filing a flight plan. I do it to get performance figures (and impress my instructor).

You're absolutely right about temperatures in METAR and TAF. Unfortunately for me, I don't usually fly very high because of clouds or short trips. Since there aren't temperatures in the 3000' FDs, I have to extrapolate between surface and 6000'.

Aviatrix said...

Yeah, when I was a flight instructor I made my students do that too. They learn to make up convincing numbers.

Julian said...

Okay my comment is not necessarily regarding TAS or photo blocks, but my it definitely has to do with flight plans. Is it necessary to add your takeoff alternate on your flight plan? I asked flight service the other day when I filed and he said no, but it would probably be a good idea should there be a com failure. obviously it wouldn't hurt to add it, but what do you think?