The VEMD and First Limit Indicator

Reading the FLI in depth

On the B3 the FLI is a large scale from 0 to 11, with 10 marked red and a yellow arc below it (Flight International's description). The arc is the take-off range, time-limited; below it is maximum continuous power.

Digital values of torque, TOT and Ng are shown beside it, and Flight International notes they turn yellow from maximum continuous power upward.

Check the markings in your own aircraft — they depend on engine and software.

Sources 1, 2

Why one needle works

Which parameter limits you changes through the day. Cold and low, torque; hot and high, TOT or Ng. Three gauges would make you work out which one matters before you could use it.

The FLI does that arithmetic continuously, at the prevailing temperature and altitude, so the needle reads as power in hand. What it doesn't show is rotor rpm — that has its own indicator — and it doesn't stop you passing a limit.

Sources 4, 5

The engine power check

The performance charts assume an engine making at least a minimum guaranteed power. The power check, run in stable flight, compares what your engine is making against that minimum.

A healthy margin means the charts apply. A shrinking margin over weeks is a trend maintenance wants to see — a dirty compressor, a worn hot section. A negative one means the charts are optimistic for that engine.

Sources 5, 6

Running the check

The check is selected manually on the VEMD. It needs stable conditions — steady power, steady flight, systems that bleed or load the engine set as the RFM requires — because it is comparing like with like.

Once the parameters have been stable for about 20 seconds, the FADEC runs the check and the VEMD shows the TOT and torque margins against a minimum engine. Record it: one result says little, a series says a lot.

Sources 1, 5

What it records

The VEMD keeps count of engine cycles and records exceedances — including rotor overspeeds — with their duration. At shutdown it shows a flight report.

An exceedance on the report is a maintenance item, not a note to self. Some require an inspection before the next flight.

Sources 1, 3

Why cycles, not just hours

Rotating parts in a turbine wear out from stress cycles as much as from running time. Each start, run to high speed and stop loads the discs and heats the hot section, and partial cycles in flight add to it.

So life-limited parts are tracked in cycles as well as hours, and the VEMD counts them for maintenance. Short sectors with many starts use up engine life faster than the hours suggest.

Sources 1, 7

The flight report

When the VEMD detects that the engine has stopped after a flight, it shows the flight report: the flight number and duration, the Ng and Nf cycles counted, and a message if a limit was exceeded or a failure detected.

Read it before you leave the aircraft. If something was exceeded, it goes in the technical log with the values shown. That is how maintenance knows to inspect — and it is in the memory whether or not you write it down.

Sources 1, 3

If a screen fails

Each lane tests itself. If one fails, its screen shows a red LANE FAILED and the other shows a yellow message asking you to switch the failed lane off (PRESS OFF1 or OFF2). Pressing it shuts the failed lane down and puts the remaining lane fully in charge.

With one lane the FLI is lost: a page with separate torque, TOT and Ng takes its place, and the power check and cycle counting stop. Separate instruments, such as the rotor and free-turbine rpm indicator, are unaffected.

Sources 1, 3