Vmca2 vs Vmco during missed approach

[spacemantan]

G'day all

I'll sum up the scenario before I ask my question.

At work we were discussing 2 engine out procedures on a particular aircraft and what the computers TOLD calculations are and FD guidance given during a missed approach. Basically the aircraft in question, upon hitting the TOGA button, will continue to descend the aircraft whilst accelerating towards Vmco. In the end this amounts to a large amount of height loss at a critical stage of an approach (assuming at DH or MAPT).

My question is, clearly the aircraft is at or above Vmca2 during the approach, so is there any reason why the aircraft can't apply max power, and as it accelerates away from Vmca2, raise the nose to a level attitude so the amount of height loss significantly decreases and then begin a slight ROC until it reaches Vmco? (long sentance).

There is clearly a reason why it doesn't do this and it is not due to the aircraft being limited in power (it has it in spades). So i'm stuck trying to figure out if there is any technical/aerodynamical/legal requirement that i'm missing.

Any thoughts?

Cheers,

Spaceman

[b707eng]

Not Sure What Vmca2 is?

On the B707 if you had 2 engines out same side and flew the approach at Vref +30, for a flap 14 landing (Vref of 136Kts+30Kts =166Kts )you were below Vmca for go around at 173Kts so you would need to continue down the slope to gain speed before you could set full go around power!

What Type are you referring to?

[rudderrudderrat]

Hi spacemantan,

With just one engine out, the aircraft must be able to GA from DA with a certain minimum gradient (2.5%?)

With 2 engines out, it probably won't climb at that minimum gradient. One way around this is to raise your commit altitude to a higher point where if you you decide to GA, you may continue down the ILS glide, and whilst you accelerate, raise the gear and flaps until the aircraft is configured as required (clean on TriStar), then climb away before reaching your DA. You'll achieve a much better gradient.

[Pugilistic Animus]

Just curious, what is " Vmco" ?

:)

[rudderrudderrat]

I'm assuming it's VMC Zero (Flap)

[spacemantan]

Hi all,

Think i just figured out the answer to my question

Vmco = Minumum climb out speed (Guaranteed obstacle clearance)

Vmca2 = Vmca for 2 engine out

Obviously it wants to get to Vmco so it can guarentee an obstacle clearance of 2.5% in the missed approach scenario.

Still the height loss that can be experienced at high operating weights in this aircraft will essentially put you on the ground from a CAT I ILS DH.

Cheers,

Spaceman

[LeadSled]

spacemantan,

Re. a four engine aircraft, Vmca2 is not a consideration for a four or three engine GA, and the aircraft limitations section of the AFM will include details of the demonstrated height loss in a GA. It is measured in tens of feet, not hundreds.

Only when you get to Cat II/III is there a potential for a touchdown during a GA, in any event, you never retract the U/C until you have a positive rate of climb.

Generally, certification does not allow for double failures (as in a second engine failure on a three engine GA in a four engine aircraft) the potential number of "double failures" is just too great.

For a B744, for example, there is no adjustment to any minima for an (1) engine out approach, compared to all engines. This is the AFM, individual NAA/airlines may impose more restrictive condition than as certified.

Further, in the event of a double engine failure, you do not commit to an approach until you know you can complete the approach, and have a landing clearance.

A good rule of thumb is "Gear down, go down" ---- from that point you are committed to a landing. You do not plan on a two engine GA on a four engine aircraft, even if, under favorable circumstances, it might be theoretically possible.

Again using the B744 as an example (and typical of FAR 25 aircraft), something called Vmco does not figure, the typical T/O climb speed to the initial cleanup altitude will be V2 + 10kt (+10 to + 25 does not significantly alter the rate/angle of climb) and for a GA, V2 is minimum, more typically V2 + 5, again to the initial cleanup altitude ---- which may be 400', but more typically 800', unless obstacles dictate a higher altitude.

Tootle pip!!

[aterpster]

Further, in the event of a double engine failure, you do not commit to an approach until you know you can complete the approach, and have a landing clearance.


Exactly!

I would add that you declare an emergency and inform the tower that under no circumstances can you possibly go around.

The way things are at the busy airports in the U.S. these days, I would probably add, "We are landing no matter what, so I trust you will help us achieve that objective safely." :)

[TheWanderer]

I would add that you declare an emergency and inform the tower that under no circumstances can you possibly go around.

Just recently I witnessed an emergency call of an aircraft just being airborne.
The tower instructed all aircraft on final to execute a missed approach while the aircraft in emergency flew a visual traffic pattern and landed safely a few minutes later.
ATC procedure seemed to not let any other aircraft takeoff or land as long as the emergency was in progress.