TAS is constant for flight level — Page 96, Lesson 90

I want to walk you through a new topic: detailed flight planning for a Medium Range Jet Transport, or MRJT. This is where we take the theory of cruise performance and apply it to real, exam-style problems. First, a critical principle: TAS is constant for a given flight level. That means if you're cruising at a specific Mach number, say 0.78 Mach, your True Airspeed stays the same throughout that cruise segment, regardless of the aircraft's changing weight as it burns fuel. The Mach number is your constant speed reference, and the flight level determines the temperature and thus the TAS. Now, the key tool you'll use is a set of tables from the CAP 697 manual, specifically the MRJT1 section. In the exam, they will not give you a page number. Instead, they will refer you to a figure number, like "Fig 4.5.3.1," and you must combine that with the given flight level to find the correct page. You need to be comfortable navigating those tables. Let's look at the general notes for these MRJT detailed flight planning tables. First, the tables use NAM — Nautical Air Miles. That means if your distance is given in NGM — Nautical Ground Miles — you will probably have to convert between them, usually by factoring in the wind component. Second, there are corrections printed at the bottom of each table page. You need to pay attention to those. Specifically, we are not interested in "thrust limiting" weights — those are a different consideration. What you should do is adjust the fuel calculated using those corrections. And crucially, you must always adjust the initial TAS for temperature deviation from ISA. Get the TAS first — you will need it anyway, and in a multiple-choice question, having the correct TAS might eliminate several wrong answers right away. Here's the core logic for fuel calculation: it is the difference in the aircraft mass at the first point compared to the mass at the second point that gives the fuel required. You start with the mass at point A, you find the fuel needed to get to point B, and the difference in mass tells you how much fuel was burned. A picture of what is going on might help, and the examples on the following pages are the sort of questions and level that EASA requires. If you ever forget the procedure, page 24 of CAP 697 MRJT1 will remind you. Now, let's look at the practice questions. These are the exact style you'll face. Question 1: An aircraft flies from A to B, a distance of 1500 NM (NGM), using Long Range Cruise (LRC) at FL330. The aircraft mass at "A" is 58,500 kg. Conditions are ISA +10°C with a 50 kt tailwind. You need to find the TAS and the fuel required. Question 2: An aircraft flies from A to B, a distance of 500 NM (NGM), at Mach 0.74 at FL290. The aircraft mass at "A" is 54,400 kg. Conditions are ISA -20°C with a 50 kt headwind. Again, find TAS and fuel required. Question 3: The aircraft mass at "A" is 51,200 kg, and the mass at "B" is 48,500 kg. Cruise is at Mach 0.78 at FL350, with ISA +20°C and a 50 kt tailwind. These are the book's practice questions — let's try them one at a time.