• Andreas

Beyond the numbers: OEI climbing speeds

Climbing with One Engine Inoperative (OEI) is certainly not an everyday situation. When the need for such a maneuver arises, adherence to procedures is critical as the margins might be quite limited in some cases. Most OEI-discussions focus on climb gradients as they are obviously of great interest. In order to achieve the advertised gradients, it is of paramount importance to fly the correct speeds. In this article we look at the different regulatory requirements governing these speeds for the OEI takeoff and the OEI go-around. Depending on the aircraft category and certification basis, definitions might vary slightly. For the purpose of this discussion, we consider transport category aircraft certified under CS25 amdt. 26 [1] and operated under EU-OPS [2].

Figure 1: If one engine fails, how fast should we fly?

OEI-climbing speed: Why it is crucial

The manufacturer will publish target speeds for the OEI takeoff scenario as well as for the OEI go-around (approach climb) case. These might look similar in terms of “numbers”, but the regulatory requirements are different [1]. The “magic number” itself is always a compromise between several aspects as depicted in Figure 2. Generally, stall-, control- and maneuver-margins represent the lower boundary for the OEI climbing speed. The performance requirements may yield an increase of the OEI climbing speed (see below). There is a fair amount of regulation concerning these speeds in the relevant certification specification [1].

Figure 2: Context of OEI climbing speed selection

OEI Takeoff speed – V2

The “takeoff safety speed” V2 is well-known amongst aircrew. The regulatory basis for V2 is summarized graphically in Figure 3 for completeness. The speed is selected by the manufacturer and usually represents an “optimum” for a given set of circumstances. In a commercial environment, the flight crew will calculate V2 for every takeoff [2].

Figure 3: V2 regulatory framework (click to enlarge)

Caution: The certification specifications mandate climb gradients and maneuvering capability. These are not required to be available simultaneously! [1]


The aircraft manufacturer will usually publish gradient loss data for banked maneuvers using 15° of bank [2]. Additionally, gradient reductions for higher bank angles may be available. In the absence of such data, the following guidelines are provided in EU-OPS [2]:

Table 1: Acceptable climb gradient adjustments in the absence of manufacturer data [2]

In normal operation (AEO), the aircraft will not actually climb at V2, but at a different speed (V2+x), which is given by the manufacturer [1].


The aircraft will arrive at the screen height at V2 if an engine failure occurs at Vef and the decision is made to continue the takeoff [1].


In many cases, it is very likely that the aircraft will be at a slightly higher speed than V2 when the engine failure occurs. The good news is, that an engine failure at V2+x is usually beneficial to climb performance as we see in Figure 4.


Improved climb performance

Typical L/D-curves for a transport category aircraft will yield higher gradients at speeds slightly higher than the minimum V2 [4]. Therefore, the manufacturer may increase V2 if climb performance is critical [1] [4]. A maximum value of V2+x is usually defined by the manufacturer and provided as an upper limit for the OEI takeoff speed.


The flight crew should be aware of the recommended speed target if an engine failure occurs at V2+x.

Figure 4: 2nd segment gradient vs. V2/Vs ratios [4]

As depicted above, higher V2 values provide better climb gradients up to a maximum ratio of V2/Vs [4]. This is why most guidance systems will command a speed slightly higher than V2, if an engine failure occurs at V2+x.


OEI Approach climb speed

A similar situation exists when a go-around is executed with one engine inoperative. The regulatory requirements are slightly different as depicted in Figure 5.

Figure 5: OEI go-around speed (approach climb) regulatory framework (click to enlarge)

The “approach climb speed” will typically be a similar value as V2 [1]. Naturally, the aircraft will have flown the approach at Vref before, so this speed is also considered.

Additionally, there rules how the transition from Vref to the approach climb speed has to be analyzed. These were modified with amdt 13 and 26 of CS25 [1]. Figure 6 contains the latest guidance provided in Cs25.

Figure 6: OEI go-around trajectory [1]

Legend for Figure 6 [1]:

Segment A: From the initiation of go-around at the decision height/altitude to the runway threshold – remain above a 1:50 (2.0 %) plane extended to the runway threshold for clearance of airport obstacles.

Segment B: From the runway threshold plus a distance defined by 40 seconds * VT_appr, not more than the distance indicated in the table below – remain above ground height.

Table 2: Distance for segment B in Figure 6 [1]

Segment C: A straight line from the end of Segment B at ground height with a gradient defined by CS 25.121(d)(1) or a steeper gradient as required by specific weather minima operational criteria, up to a height, H1 – remain above the line.


Where:

VT_appr is the true airspeed for the normal recommended AEO approach speed in zero wind at the flight condition being assessed (not more than VREF + 9.3 km/h (5 kt) CAS).

H1 is the height above the runway elevation where the airplane has achieved the approach climb configuration and stabilized on the approach climb speed out of ground effect (1x the wingspan), not less than the height at which the go-around was initiated.


Again, a slight increase in speed may provide better climb gradients, but this time there is a “hard” regulatory limit of 1.4 Vsr for the approach climb speed [1].


The regulatory framework can be quite complex when it comes to OEI operating speeds. Sticking to the numbers combined with a healthy understanding of aircraft performance provides a good basis for a safe outcome.


Revision/20210904


References

[1] EASA, Easy Access Rules for Large Aeroplanes (CS-25), amdt. 26, 2021

[2] EASA, Easy Access Rules for Air Operations (EU-OPS), July 2021

[3] Blake & Performance Training Group Flight Ops Engineering Boeing, Jet Transport Performance Methods, Boeing, 2009

[4] AIRBUS, Getting to grips with aircraft performance, 2002