what speed when both engines fail

[lomapaseo]

Check this out, really interesting:

Loss of Thrust on Both Engines

Of course the best answer to the thread title

The historical problems have been where the engines are out of the restart envelop and have already spooled down. (due to extremely high pilot workload sorting out what happened).

Recovering from this has more often resulted in hung/hot starts and thermal damage to the turbine rendering any further restart hopeless.

Not a critism of the crew but just food for thought in advance

Also give a thought that if you are at high altitude (volcanic ash etc.) that the recognition of a sucesseful restart will be different than on the ground. The engines RPM and EGT will increase very slowly. While Overtemp EGT is a reliable indication of lack of sucess

[misd-agin]

If you've given up on the engine restart attempts QRH (7.16 in ours) had the dual engine flameout speed.

Quick estimate for a 737-800NG w/winglets -

Vref 15 +60 kts averages L/D +/- 2.5 kts.

Gross weight in lbs + 75 kts averages L/D +/- 4 kts.

Doesn't work at high altitudes, but with both engines out you won't be in the 30's for very long.