In landing configuration, at a heavy weight right after takeoff, airliners don't have enough power to fly level. This was a training event that was introduced to demonstrate that at heavy weights (ie immediate return scenario), even at max power you'd be unable to hold level flight while in landing configuration while single engine. Rule of thumb "no gear down until you're going downhill" (ie descent patth without level offs).
That's not a universal truth, as a quick look at the performance figures contained in the QRH for e.g; a 777 will reveal.
BTW I take it we think this is a series of surges, rather than a fire ( I think that's the point Compass Call is making).