## CNAV Members Only Tips

We have discovered the tolerance or accuracy of Take-off Performance Charts is as little as 15m for CASA exams (which is ridiculous).

This equates to a final TODR accuracy of 0.66mm on the right side of the chart, which is 1/3rd of one box.

So here's what you MUST do.

- Buy a pair of cheap strong reading glasses from the chemist or eBay for $5.50 delivered.
- Use a sharp mechanical pencil and mark both ends of each line then check with the magnifier. If accurate draw the line. If not remark. Repeat this for every line.
- Where lines follow curves, use 2-3 segments to follow the curve and the above process.
- Check each step for deadly accuracy.
- Deliberately make a mistake and ask for a second work booklet. Redraw it and compare.
- Do this in two work booklets. If your decision is a line ball, double check everything and adjust.
- Note: if you get 3 TO chart questions at 2 marks each out of your 56 questions. That right there is 12% lost if you get them wrong. That’s 40% of your possible lost marks on one topic. So it is vital you practice these over and over with precision accuracy.

NOTE: In MOST cases the CASA question will very clearly state if they want you to use the slope or not. But in some its more subtle.

**Wording From CAO 20.7.4 4.1**

Example **when to use the CAO** and ignore any slope that is NOT in excess of 1%.

Question would say: Aerodrome alpha has 1% slope, OAT 10°C, QNH 1016, surface long dry grass etc....calculate TODR.

**In this case use a slope of 0%**

Example **when NOT to use the CAO** and to include a slope NOT in excess of 1%. .

Question would say: Given 1% slope, OAT 10°C, QNH 1016, surface long dry grass etc....calculate TODR... or

Given the following: 1% slope, OAT 10°C, QNH 1016, surface long dry grass etc....calculate TODR...

**In this case use a slope of 1%**

In some cases a question will also state Runway 21 slopes down 1% to the South and Runway 09 slopes up 1%.

AWIS 220/10

Using the above, calculate TODR.

**In this case use a slope of 1%**

However if it said:

Aerodrome Alpha Runway 21 slopes down 1% to the South and Runway 09 slopes up 1%.

AWIS 220/10

Calculate TODR.

**In this case do not use a slope of 1%, use the CAO and ignore the 1% slope, because they have not instructed you to use information given. **

**For CPL and ATPL only** - You also need to be very careful of the aircraft weight above or below 5700kg. For aircraft below 5700kg the CAO says to ignore slopes not greater than 1% for both takeoff and landing. But the CAO for aircraft above 5700kg has not mention of 1% for takeoffs, it does however say to ignore slopes not greater than 1% for landings.

See CAO's 20.7.4 and 20.7.1b

Safe endurance is what remains after all the reserves have been subtracted from the fuel available.

Safe endurance is the amount of fuel that you can plan to burn safely in flight and still have the required reserves in tact.

**CPL Will Get Asked Like This**

RPL & PPL should read this section to understand it, but you will get asked like the question below the blue heading.

Consider a CHTR flight to an airfield which requires INTER holding in a small, piston engine aeroplane.

Fuel at start = 186 mins = 12 (start & taxi) + 90 (Trip Fuel) + 9 (10% Variable reserve) + 30 (INTER holding) + 45 (Fixed Reserve)

Now if you were told that you had 186 mins fuel at start, then once you subtract all the elements (highlighted in red), then you should be back at 90 mins as the safe endurance for this flight.

Lets assume a 10% variable is required for this question.

186 mins =** 12 (start & taxi) + ?? (Trip Fuel) + ? (10% Variable reserve) + 30 (INTER holding) + 45 (Fixed Reserve)**

186 – 12 – 30 – 45 = Trip fuel + 10%

99 = 110% of Trip Fuel

Trip Fuel = 99 / 1.1 = 90 mins

Safe endurance = 90 minutes

**Now lets consider the same CHTR question without an alternate due to weather. **

Fuel at start = 186 mins = 12 (start & taxi) + 90 (Trip Fuel) + 9 (10% Variable reserve) + 45 (Fixed Reserve) + 40 (To Alternate)

186 mins =** 12 (start & taxi) + ?? (Trip Fuel) + ? (10% Variable reserve) + 45 (Fixed Reserve) + 40 (Alternate) **

186 – 12 – 45 - 40 = Trip fuel + 10%

89 = 110% of Trip Fuel

Trip Fuel = 89 / 1.1 = 80.9 mins

Safe endurance = 81 minutes

Remember: As per Table 1 in CAAP 234-1(2) the variable reserve must not be less than 5 minutes for CHTR and RPT.

See note re consumption rates below.

**For RPL & PPL & CPL **

**Now lets consider the same PRVT question without an alternate due to weather and not requiring a variable reserve and only requiring a 30 min fixed reserve. **

Fuel at start = 186 mins = 12 (start & taxi) + 90 (Trip Fuel) + 30 (Fixed Reserve)

186 mins =** 12 (start & taxi) + ?? (Trip Fuel) + 30 (Fixed Reserve)**

186 – 12 – 30 = Trip fuel

144 min = Trip Fuel

Safe endurance = 144 minutes

**Now lets consider the same PRVT question with an alternate due to weather and not requiring a variable reserve and only requiring a 30 min fixed reserve. **

Fuel at start = 186 mins = 12 (start & taxi) + 90 (Trip Fuel) + 30 (Fixed Reserve)

186 mins =** 12 (start & taxi) + ?? (Trip Fuel) + 30 (Fixed Reserve) + 30 (INTER holding)**

186 – 12 – 30 - 30 = Trip fuel

114 min = Trip Fuel

Safe endurance = 114 minutes

**An important note on fuel consumption rates: **

Trip Fuel and fuel to the alternate are calculated at cruising fuel consumption rate, which will be given in the question.

Holding , INTER and TEMPO are calculated at the holding fuel consumption rate, which will be given in the question.

**The Big Mistake**

A huge mistake people make repeatedly is not knowing how to calculate a fuel reserve and trip fuel.

If after deducting all holding , startup and taxi fuel you are left with 89L fuel on board for the trip fuel plus reserves, then you must divide it by 1.1 for a 10% reserve or 1.15 for a 15% reserve.

The reason is because the fuel left on board is made up of 100% of trip fuel + 10 % variable reserve = 110 parts, not 100 parts of fuel.

E.g. 146 min FOB – 12 min startup and taxi – 45 min fixed reserve = Trip fuel + 10%

89 minutes = Trip Fuel + 10% Variable Reserve

Trip Fuel = 89 / 1.1 = 80.9 mins

Trip Fuel or Safe endurance = 81 minutes (close enough we can round up).

For a 15% Variable Reserve the calculation would be:

E.g. 146 min FOB – 12 min startup and taxi – 45 min fixed reserve = Trip fuel + 15%

89 minutes = Trip Fuel + 105% Variable Reserve

Trip Fuel = 89 / 1.15 = 77.4 mins

Trip Fuel or Safe endurance = 77 minutes (round down for safety).

Candidates **MUST READ** the question.

Sometimes the question will say, ROUND UP to the next whole (L, Kg, USG, m nm etc.).

Usually it will say this to ensure that we have slightly more fuel than the absolute legal minimum and that you comply with the laws.

Other times it may say ROUND DOWN, for example with payloads so that we don’t overload and damage the aircraft or breach legal maximums.

In the absence of any specific instruction with rounding, then it generally won’t matter, as long as the rest of the working is accurate and you would round on the final step using traditional rounding techniques.

Unless you have been instructed otherwise, or a requires rounding (e.g. the density height workings) do not round until the answer when doing multi step calculations. Rounding too early only magnifies errors of rounding.

If in doubt always leave rounding until the end of your working.

If something has 10 decimal places you will definitely not need to keep all 10.

How many decimal places you keep may vary from question to question.

With a Weight & Balance questions keep at least 4 decimal places.

When calculating an ETI for a flight sector,1 or 2 decimal places will be fine.

Before you begin calculations and any rounding read what the question says and if it is a multiple choice, check how close the answers.

If you are uncertain, or confused DO NOT round!

**From Part 61 MOS Volume 3**

**5.2 Flight planning**

(g) demonstrate accuracy in computations:

**(i) HDG +/- 5 ^{o}, ETI +/- 2 mins; and**

**(ii) fuel and endurance +5%.**

ERSA states SG of AVGAS is 0.72kg per L.

The RPL PPL CPL workbook on the Charlie loading system states it is 0.71 kg per L.

So which do you use and why?

0.71 when doing charlie and 0.72 for everything else.

Gavin from CASA has stated that using 0.71 or 0.72 will be insufficient difference to get a question wrong in PPL and CPL exams.