TAS calculation in climb

[Pole Hill]

I cannot get my head around this. I've tried my Ground School's Website but nobody has replied. :{
I am using the CAP697 Flight Planning Manual, and am looking at 'Time, Fuel, and Distance to Cruise Climb' graph. Assuming NIL WIND.

The calculation of the climb TAS is performed by:
1. Taking the IAS from the Climb Graph for the aircraft in the manual
2. Calculating the mean climb height and temperature
3. Using the CRP5 to calculate TAS with the known variables above. This works ok, and I get the correct result.

The graph also gives the climb distance, the time and fuel required. So, as a alternative method, I thought that using the climb distance (NAM) and time from the graph, I could calculate TAS (speed = distance/time). I don't get the correct answer, I get something totally different. Why?

Thanks,
POL

[Keith.Williams.]

If you use horizontal distance/time you will get your groundspeed. This is a horizontal speed.

If you use your CAS and correct it for atmospherics you will get your TAS. This is your speed up a sloping flight path so it is not the same as your groundspeed.

The diference between the two will depend upon your climb gradient. The COS of the climb angle = groundsped/TAS

[lionco]

Pole you will find that if you use the conditions of two thirds of the climb, the answer will be much closer for average TAS in the climb.
Due to a lower RoC you will spend more time in an area where the TAS is lower.
Kieth is of course correct about the slant distance, although the difference slant to ground for short SEP & MEP climbs in still air is very small.


:ok:

[Genghis the Engineer]

Pythagoras, that famous Greek groundschool instructor said:-

- Take TAS and square it
- Subtract the square of the climb rate (in the same units).
- Take the square root of the answer, and that's your equivalent TAS in level flight.
- Then factor as usual to give groundspeed.


(Incidentally, and this has nothing to do with pythag, if you are trying to be really precise, convert from IAS to CAS and work out TAS from there - there should be a graph or table for this in the POH somewhere. The difference can be as much as the greater of 5% or 5 knots in a certified aeroplane).

G

[High Wing Drifter]

Pole Hill,

I could be mistaken in the case of your specific calculation. But I am sure that the method is listed in CAP697 and it may not be in the MRTJ bit. In fact the exam will require you to use that method to get the answer they are looking for...especially so if the answer options are closely spaced.

And another thing. Don't forget to account for the height of the departure aerodrome.

[mad_jock]

Remember as well that performance changes with altitude.

So you won't have a constant IAS or climb rate through the whole of the climb.

The crap5 answer will be the correct nothing changes everything is perfect answer.

The pref figures which were real ones i believe. Will take into account all the horrible stuff that real airplanes do.

So only adivice is use the pref tables when asked and use crap5 when not asked. Its all a load of bollocks anyway just to pass the exams don't worry about it.

In real life if you ever have to plan a route (which would be very unusall) you would use the graphs. The result would be completly incorrect but you would be legal.

If its any help I did all my perf for the ATPL's the next time I looked at perf it was for 1 day on my type rating. Never looked at one since unfortunatly ( which is mainly due to the UK MSA being max 5600 and all the runways we are operating into are classed as long so no perf needed for our aircraft)

MJ

[Pole Hill]

Thank you everyone for replying, I appreciate your feedback.:ok:

These calculations are for the SEP and MEP aircraft. I remember working out the TAS from mean alt/temp using the CRP5 and it being 130kts. I worked out TAS using climb distance and climb time and got 105kts. I appreciate Pythagorus' theroy, but my vertical speed cannot be 77kts.

When I get home tonight I'll get out the question details and post it on here.

Strangley though, I thought I should be using 2/3 of climb conditions for mean alt/temp, but the notes state that I should be using 1/2 of the climb alt. Maybe the 2/3 of climb alt is only used for climbs in MRJT?

Thanks,
POL

[Pole Hill]

I know this is for a descent, but the same problem applies..

The question is:
"Find the Fuel used, the Time and the Distance covered in a descent from Pressure Altitude 16000ft, where the OAT is -16'C, to an Aerodrome at Pressure Altitude 2000ft, OAT +10'C in a 20kt headwind."

Relevant bits from the graph (I have taken account of aerodrome pressure alt):
Time = 14min
Dist (NAM) = 37

If you work out TAS in the descent on the CRP5, using 145KIAS as stated on the graph, mean pressure alt of 9000ft and mean oat of -3'C, answer is 166Kt.

But if you do the simple formula speed = dist (NAM)/time, speed or TAS = 159Kts.

If you assume 166kt is the slant TAS, and 159Kts is the horizontal component of TAS, the vertical component of TAS is 48Kts. Therefore, this cannot be the case. :{
Why is there a TAS disparity? Shouldn't the TAS calculated by both methods give just about the same answer.:confused:

Any help much appreciated. :ok:
Thanks,
POL.

[High Wing Drifter]

Pole,

I get results slightly different from you and more inkeeping with the CRP. I get 38 NAM not 37 wich = 164kts TAS which is a mere 2kts different from the mean alt/temp TAS of 166kts. Looking at the graph it actually looks more like 38.5 NAM which is 165kts. It is not possible for everybody to get the same pinpoint numbers with these graphs. Just follow the procedure you were taught and you will get the right answers in the exam.

[oxford blue]

The usual rule in planning climbs or descents when there is no appropriate page in the Flight Manual or Operating Data Manual is to use the climb (or descent) CAS and the FL, temperature and W/V for the 2/3 climb FL or the 1/2 descent FL. However, whilst this is generally good and will serve you well without being too much in error for an unknown aircraft, where there is a manual which gives specific climb and descent data for your particular aircraft, then you use the manual. (Or CAP 697 for the JAA ATPL FP exams). After all, it comes with the authority and stamp of approval of the manufacturer - it's not just some gash generally correct formula. The answers will be close whichever method you use, but not identical.

Keith is theoretically right when he says that TAS is up the slope but G/S is on the level, but practically the effect is negligible and the effect of the cosine of the climb angle should not be taken into account in Flight Planning. It might be appropriate for Performance.

Always use the data in the graphs. You don't actually need the TAS in the climb or descent and you should not calculate it when converting NAM into NGM in climbs or decsents for SEP and MEP. Extract the NAM from the graph then convert it into NGM by use of the following formula:

NGM = NAM + (W/C x climb time/60)

That is what the JAA are expecting to see as the answer.

Alles klar?

[High Wing Drifter]

Oxford's forumula is published in the CAP. However it is only published in the MRTJ section. As I was told by Bristol GS this is irrelevant and should be used for relevant problems for SEP and Multi too.

[Pole Hill]

Looks like I am making something out of nothing.
Thank you HWD for taking the time to go through the question, and thank you Oxford Blue for the explanation.:ok:
POL.