The conventional advice when using abbreviations in your essay is to write them out in full with the abbreviated form you intend to use later in the text. Although this is better than no explanation, these defined abbreviations are often the ones to avoid. Each time you define another abbreviation, you set a memory test for your reader. We have tested this by asking people to read a two-page document with two abbreviations explained on the first page. When they turned the page and read the abbreviations, we asked them what they stood for. Only one person got one correct answer - a 95% failure rate. When writing an essay, keep your abbreviations under control.
Use the abbreviation without explanation, if everyone knows its meaning. o IBM, USA, Washington DC, BMW, PhD
Note: avoid using an abbreviation if it's commonly understood as one term and you mean another. For example, do not use PC for politically correct or for Privy Council as most people think of it as meaning personal computer.
So, when you draft your essay, use Accident Data Recorder shorten it to Recorder rather than ADR when you use it later. Write alt in the full form of altitude, prefer satellite communications rather than sat comms and sea level to SL. You'll find no one misses your shortened forms and your writing style will sound more natural.