Is it possible that derated thrust takeoffs are safer than normal takeoffs?

Question

The reason for my arguments here are (assuming that you have sufficient runway distance):

That with less thrust, less force should also be required to hold the aircraft as desired, and possibly the aircraft should be easier to control and fly when the engine fails than if it does so with the thrust set to full blast.

An engine event, such as a bird strike, might (?) be less likely to have catastrophic consequences for e.g. the turbine blades when they are not running at full thrust and their design limits.

Never having actually commanded an aircraft, are any of these ideas correct?

Answer 1

The reasons presented at ground school for the benefits of de-rated thrust takeoffs were twofold:

• less chance of catastrophic engine failure during takeoff
• less overall wear between overhauls

It is from this standpoint that reduced thrust takeoffs are safer.

As you point out, these are only authorized when the weights, temperature and runway length allow, but for the EMB-145 virtually every takeoff we did was eligible for reduced thrust.

You point about ease of control with an engine out is countered by engine out procedures which involve setting max thrust on the remaining engine (for two-engine ops). During takeoff roll in the EMB-145XR, this puts the other engine at 117% power. With the exception of flying near $V_{MC}$ (for light twins) the extra thrust is a benefit and not a hindrance to control and if this is at or after $V_1$ you'll want the thrust to go flying.

Your question about a bird strike is going to go one of two ways:

1. the bird hits the N1 fan and goes through the bypass ducting
2. the bird hits the N1 fan and is ingested into the N2 compressor stages

In either scenario a reduced thrust vs full thrust takeoff is not going to make a huge difference, but scenario 2 will be far more destructive to the engine than scenario 1.

Answer 2

An airline policy of doing reduced thrust takeoffs is considered to improve safety in the longer term by reducing the probability of an engine failure, due to reduced cumulative wear on the engines and reduced engine stress during each takeoff.

The primary motivation for reduced thrust takeoffs however is cost. Engine life can be significantly extended by use of this policy. What "kills" an engine is peak engine temperature (EGT or ITT). Each degree of thrust reduction through assumed temperature method or derated takeoff makes a meaningful difference in peak EGT for that takeoff, and leads to a large cost saving in the long term.

Doing a reduced thrust takeoff does not reduce your safety margin in the event of an engine failure, since the engine-out takeoff profile will be calculated for that thrust value, ensuring that safe obstacle clearance will exist for each segment of the profile. In addition to this however, maximum thrust is still available to the pilot at any point should he desire more thrust. Additionally, from a handling point of view an engine failure will be easier to control at a lower takeoff power setting than at maximum takeoff thrust.

Reduced thrust takeoffs is the industry standard and I would estimate that less than one in a thousand of the takeoffs I do is with maximum thrust. In fact we have to write it up in the technical log every time a maximum thrust takeoff is done.