Question: Unable to convert to an explicit first-order system?

Hello everybody,

   I have to solve the following differential equation numerically

dsys1 := {0.27e-2*(diff(x(t), [`$`(t, 2)])) = -3*10^(-4)*sqrt((diff(x(t), t))^2+(diff(y(t), t))^2+(diff(z(t), t))^2)*(diff(x(t), t))-(2.25*10^(-4)*8)*(diff(y(t), t)), 0.27e-2*(diff(y(t), [`$`(t, 2)])) = -3*10^(-4)*sqrt((diff(x(t), t))^2+(diff(y(t), t))^2+(diff(z(t), t))^2)*(diff(y(t), t))+(2.25*10^(-4)*8)*(diff(x(t), t)), 0.27e-2*(diff(z(t), [`$`(t, 2)])) = -3*10^(-4)*sqrt((diff(x(t), t))^2+(diff(y(t), t))^2+(diff(z(t), t))^2)*(diff(z(t), t))-0.27e-1, x(0) = 0, y(0) = 0, z(0) = 0, (diff(x(t), t))(0) = 15, (diff(y(t), t))(0) = 1, (diff(z(t), t))(0) = 0}

sol1 := dsolve(dsys1, numeric, method = rkf45, range = 0 .. 10)

I don't know why it says : Error, (in DEtools/convertsys) unable to convert to an explicit first-order system

Help pleaaaase!