VTOSS VERSUS VY REVISION

1. VTOSS VERSUS VY
GUILLERMO SÁENZ SAN BARTOLOMÉ
ONSHORE/OFFSHORE SAR CAPTAIN

2. AW-139 40 KIAS
< 10000 ft Hp 80 KIAS
>10000 ft Hp 70 KIAS
BK-117 45 KIAS 65 KIAS (No varía en el RFM)
EC-135 40 KIAS
< 15000 ft PA 65 KIAS
>Linear decrease according
RFM
AW-109 30KIAS 60 KIAS (No variation RFM)
• VTOSS, take off safety speed. Although it
is not a fixed speed, since it increases
with altitude, actually the RFM it comes
as fixed, usually in the range of 40/45
KIAS; This speed is used to achieve the
necessary regime with power of 2.5
minutes (OEI) of 100 fpm up to 200 ASL.
For our PC2 calculations will be used as
target speed.
• VY (best rate of climb) is the speed
indicated (IAS) that provides the best
speed of climb. It is not a fixed velocity
since it decreases from a certain height
and normally in a linear way, although
in some models and RFM they refer to it
as constant (BK-117, AW-109).

3. POWER-SPEED GRAPH
• FROM THE GRAPH, IT IS OBSERVED THAT AS THE
ALTITUDE INCREASES, VTOSS INCREASES AND VY
DECREASES, ARRIVING A MOMENT IN WHICH
BOTH ARE EQUAL, THAT WILL HAPPEN WHEN THE
ABSOLUTE CEILING OF THE HELICOPTER IS
REACHED, DIFFERENT FROM THE CEILING OF
SERVICE, THAT BY DEFINITION STILL SUFFICIENT
POWER REMAINS TO MAKE AN ASCENT WITH 50
FPM

4. VTOSS versus VY
• ASCENDING TO VTOSS ALLOWS THE PILOT TO
MAXIMIZE THE ALTITUDE GAINED PER UNIT
DISTANCE, IN OUR CASE BY NM TRAVELED. THE
DISTANCE TRAVELED (NM) IS SACRIFICED TO GAIN
ALTITUDE.
• ASCENDING TO VY ALLOWS THE PILOT TO
MAXIMIZE THE ALTITUDE GAINED PER UNIT OF
TIME, IN OUR CASE MEASURED IN MINUTES.
THEREFORE, VY, ALLOWS THE PILOT TO MAXIMIZE
ITS RISE "SACRIFICING" THE SHORTEST POSSIBLE
TIME.

5. VTOSS IS THE SPEED USED IF WE HAVE TO GAIN
ALTITUDE OVER A RELATIVELY SHORT DISTANCE
DUE TO NEARBY OBSTACLES, AS IT GAINS THE
HIGHEST ALTITUDE OVER A SHORTER DISTANCE.

6. VTOSS VS VY
MAKING A COMPARISON BETWEEN THE CLIMB
RATES TO VTOSS AND VY IN OEI FOR THE EC-135
P2 +, AT 2000' AND 30ºC, FOR DIFFERENT
WEIGHTS

7. VTOSS VS VY
• AW-139 WE CAN EXTRACT DATA FROM
SECTION 4 (PERFORMANCE), SUPPLEMENT
12 (CAT A), SUPPLEMENT 50 (INCREASED
GROSS WEIGHT) AND SECTION 9 OF THE
RFM. IN THE FOLLOWING TABLE WE
COMPARE THE REGIMES FOR VY UNDER
DIFFERENT CONDITIONS FOR BOTH AEO AND
OEI (2000', 30 ° C, 100% RPM):

8. VTOSS VS VY
• IF WE EXTRACT DATA COMPARING THE
MEAN HEIGHT GAIN IN 100 FT, WHICH GIVES
US AN ASCENT GRADIENT WITH THE
CONDITIONS WE ARE USING IN OUR
CALCULATIONS (2000'AND 30 ° C):

9. VTOSS VS VY
From the data we draw the following conclusions:
• Gradient greater VTOSS than VY.
• The greater the weight, the smaller the difference in the gradient.
• With this data, considering a take-off in a confined area or with
nearby obstacles, it will be necessary to plan to climb with VTOSS
since the important thing is to reach the safety height on the obstacle.
• VTOSS is the speed that must be kept in mind in every takeoff in a
twin engine. If we review all the profiles and emergency procedures of
motor failure after the TDP in CAT A PC1 or DPATO to PC2, we will see
that it is the speed to be reached after engine failure, since the main
thing is to gain height, without traveling a great distance to save the
obstacles in take-off path.